<PAGE>
                                                                   Exhibit 10.21

                    AMENDMENT NO. 4 AND WAIVER dated as of May 10, 2002 (this
               "Amendment"), to the Credit Agreement dated as of June 28, 1999,
               as amended and restated as of July 21, 1999, as amended (the
               "Credit Agreement"), among Knowles Electronics Holdings, Inc.,
               formerly known as Knowles Electronics, Inc., a Delaware
               corporation (the "Parent Borrower"); the financial institutions
               party thereto as Lenders (the "Lenders"); JPMorgan Chase Bank, as
               successor to The Chase Manhattan Bank, as administrative agent
               (in such capacity, the "Administrative Agent") and Morgan Stanley
               Senior Funding, Inc., as Syndication Agent.



          A. The Lenders and the Issuing Bank have extended credit to the
Parent Borrower, and have agreed to extend credit to the Parent Borrower, in
each case pursuant to the terms and subject to the conditions set forth in the
Credit Agreement.

          B. The Parent Borrower has requested that the Required Lenders agree
to amend and waive certain provisions of the Credit Agreement as set forth
herein. The Required Lenders are willing to agree to such amendments and waivers
pursuant to the terms and subject to the conditions set forth herein.

          C. Capitalized terms used but not defined herein have the meanings
assigned to them in the Credit Agreement as amended and waived hereby.

          Accordingly, in consideration of the mutual agreements herein
contained and other good and valuable consideration, the sufficiency and receipt
of which are hereby acknowledged, the parties hereto agree as follows:

          SECTION 1. Amendments.  The Credit Agreement is hereby amended as
follows:

          (a) The table in the definition of the term "Applicable Rate" in
Section 1.01 of the Credit Agreement is replaced with the following table:

<Table>
<Caption>
                                                           Eurodollar
                                          ABR Revolving/   Revolving/                    Eurodollar
                                            Tranche A      Tranche A     ABR Tranche B    Tranche B    Commitment
          Leverage Ratio:                    Spread          Spread          Spread         Spread      Fee Rate
------------------------------------------------------------------------------------------------------------------
<S>                                          <C>             <C>             <C>            <C>           <C>
Category 1
Greater than or equal to 4:50 to 1:00         2.50%           3.50%           3.50%          4.50%         0.50%

Category 2
Greater than or equal to 3.75 to 1.00         2.25%           3.25%           3.50%          4.50%         0.375%
and less than 4.50 to 1.00

Category 3
Greater than or equal to 3.00 to 1.00         2.00%           3.00%           3.50%          4.50%         0.375%
and less than 3.75 to 1.00

Category 4
Greater than or equal to 2.25 to 1.00         1.75%           2.75%           3.50%          4.50%         0.375%
and less than 3.00 to 1.00

Category 5
Less than 2.25 to 1.00                        1.50%           2.50%           3.50%          4.50%         0.375%
</Table>
<PAGE>
                                                                               2

          (b) Section 1.01 of the Credit Agreement is hereby amended by the
addition of the following definitions in the appropriate alphabetical position:

          "Investor Funding Agreement" means the Investor Funding Agreement
     dated as of May 10, 2002, among Key Acquisition L.L.C., the Parent Borrower
     and the Administrative Agent.

          "Parent Investment" means the investment required to be made by one or
     more of the Sponsors pursuant to the Investor Funding Agreement.

          "Required Additional Capital" means (a) Indebtedness of the Parent
     Borrower permitted pursuant to clause (xii) of paragraph (a) of Section
     6.01 or (b) capital stock of the Parent Borrower, including preferred stock
     or the Parent Investment, in each case incurred or issued after May 10,
     2002 and resulting in cash proceeds to the Parent Borrower of at least
     $10,000,000; provided that the terms and conditions of such Indebtedness or
     capital stock are reasonably satisfactory to the Administrative Agent.

          (c) Section 2.05 of the Credit Agreement is hereby amended by deleting
the reference "Section 2.11(b)" in each instance it appears in paragraph (j)
thereof and substituting in lieu thereof the following:

     paragraph (b) or (g) of Section 2.11, as applicable

          (d) Paragraph (c) of Section 2.11 of the Credit Agreement is hereby
amended by adding the following sentence after the end of the first sentence of
such paragraph: "Notwithstanding the proviso to the preceding sentence, the
aggregate amount of Net Proceeds that the Parent Borrower may elect to apply to
acquire assets in lieu of prepayment, as contemplated by such proviso, shall not
exceed $2,000,000 in the aggregate after April 1, 2002."

          (e) Section 2.11 of the Credit Agreement is hereby further amended by
adding at the end thereof the following:

          (g) In the event that, on the date that any proceeds of any Required
     Additional Capital are received, the Domestic Revolving Exposures exceed
     $8,250,000, then on such date the Parent Borrower shall prepay Domestic
     Revolving Borrowings or Domestic Swingline Borrowings (and, if repayment of
     all such Borrowings is insufficient to reduce the Domestic Revolving
     Exposures to $8,250,000, deposit cash collateral in an account with the
     Administrative Agent pursuant to Section 2.05(j)), in an aggregate amount
     for such repayments and deposits equal to the lesser of such excess and the
     amount of such proceeds.

          (f) Article V of the Credit Agreement is hereby amended by adding at
the end thereof the following;

          SECTION 5.15. Payment of Interest on Subordinated Debt. The Parent
     Borrower will pay the interest due April 15, 2002 with respect to the
     Subordinated Refinancing Debt immediately following the Amendment Effective
     Date (as
<PAGE>
                                                                               3

          defined in Amendment No. 4 and Waiver dated as of May 10, 2002, to
          this Agreement).

          (g)  Section 6.01 of the Credit Agreement is hereby amended by (i)
deleting the word "and" appearing at the end of clause (x) of paragraph (a)
thereof, (ii) deleting the period at the end of clause (xi) of paragraph (a)
thereof and substituting in lieu thereof the following: "; and" and (iii) adding
following clause (xi) of paragraph (a) thereof the following:

          (xii) Indebtedness of the Parent Borrower in an aggregate principal
     amount not exceeding $10,000,000; provided that such Indebtedness is
     subordinated to the Obligations, and has other terms and conditions,
     reasonably satisfactory to the Administrative Agent.

          (h)  Section 6.04 of the Credit Agreement is hereby amended by adding
at the end of clause (a) thereof the following:

     ; provided that no Permitted Acquisition will be permitted after April 1,
     2002;

          (i)  Section 6.09 of the Credit Agreement is hereby amended by adding
the following sentence at the end of such Section:

     Notwithstanding clause (e) above or any other provision of this Agreement,
     the Parent Borrower will not, and will not permit any Subsidiary to, make
     any payments of fees to any of the Sponsors (whether under the management
     agreement referred to in clause (e) above or otherwise) after April 1,
     2002.

          (j)  The table in Section 6.12 of the Credit Agreement is replaced
with the following table:

<Table>
<Caption>
                    Period                               Ratio
                    ------                               -----
<S>                                                    <C>
June 30, 2002 through March 31, 2003                    1.45 to 1.00
April 1, 2003 through March 30, 2004                    2.25 to 1.00
March 31, 2004 through March 30, 2005                   2.50 to 1.00
March 31, 2005 through March 30, 2006                   2.75 to 1.00
March 31, 2006 through March 30, 2007                   3.00 to 1.00
March 31, 2007 and thereafter                           3.25 to 1.00
</Table>

<PAGE>
                                                                               4

          (k)  The table in Section 6.13 of the Credit Agreement is replaced
with the following table:

<Table>
<Caption>
                  Period                                               Ratio
--------------------------------------------                        ------------
<S>                                                                 <C>
April 1, 2002 through September 29, 2002                            6.75 to 1.00
September 30, 2002 through December 30, 2002                        6.65 to 1.00
December 31, 2002 through March 30, 2003                            6.45 to 1.00
March 31, 2003                                                      5.85 to 1.00
April 1, 2003 through March 30, 2004                                4.00 to 1.00
March 31, 2004 through March 30, 2006                               3.75 to 1.00
March 31, 2006 and thereafter                                       3.50 to 1.00
</Table>

          (l)  Section 6.13 of the Credit Agreement is hereby further amended by
the addition of the following at the end of such Section:

          On or prior to April 10, 2003, the Parent Borrower shall deliver to
     the Administrative Agent a certificate of a Financial Officer to the effect
     that the Leverage Ratio on and as of April 1, 2003, complies with the
     requirements of this Section. Failure to deliver such certificate shall
     constitute an Event of Default.

          (m)  Section 6.14 of the Credit Agreement is hereby amended by
deleting the amount "$20,000,000" appearing in paragraph (a) thereof and
substituting in lieu thereof the amount "$15,000,000".

          (n)  Article VII of the Credit Agreement is hereby amended by (i)
deleting the word "or" appearing at the end of paragraph (m) thereof, (ii)
adding the word "or" at the end of paragraph (n) thereof and (iii) adding
following paragraph (n) thereof the following:

          (o)  (i) the Parent Borrower fails to receive the Required Additional
     Capital (other than the Parent Investment) on or prior to September 3, 2002
     or (ii) the Parent Investment is not made on or prior to the Funding Date
     (as defined in the Investor Funding Agreement);

          SECTION 2. Decrease in Revolving Commitments. The parties hereto
agree that on the Amendment Effective Date the Domestic Revolving Commitments
shall be reduced by $6,750,000 to $18,250,000, with the result that the
Revolving Commitments are also $18,250,000, and that such reduction shall be
allocated pro rata among the Lenders based on their Domestic Revolving
Commitments.

          SECTION 3. Waivers. The Required Lenders hereby waive compliance by
the Parent Borrower with the provisions of (a) paragraph (a) of Section 5.01 of
the Credit Agreement with respect to the financial statements for the fiscal
year ended December 31, 2001, provided that the waiver contained in this clause
(a) shall terminate if such financial statements are not delivered on or prior
to the date that is 30 days

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                                                                               5

following the Amendment Effective Date (as defined below), (b) clause (iii) of
paragraph (a) of Section 6.01 of Credit Agreement with respect to the
refinancing of the Indebtedness listed on Section 1.a of Schedule 6.01 to the
Credit Agreement with Indebtedness of Ruwido Austria GmbH in favor of Ernst Bank
Vienna in the aggregate principal amount not exceeding Euro 4,200,000; provided
that following June 30, 2002, the aggregate principal amount of such
Indebtedness shall not exceed Euro 4,000,000, (c) Section 6.12 of the Credit
Agreement for the period ended March 31, 2002, (d) Section 6.13 of the Credit
Agreement for the period from January 1, 2002 through March 31, 2002, and (e)
paragraph (g) of Article VII with respect to the failure to pay the interest due
April 15, 2002 on the Subordinated Refinancing Debt prior to the Amendment
Effective Date.

          SECTION 4. Representations and Warranties. The Parent Borrower
represents and warrants to the Administrative Agent and to each of the Lenders
that:

          (a)  This Amendment has been duly authorized, executed and delivered
     by it and constitutes the legal, valid and binding obligation of the
     Parent Borrower, enforceable against the Parent Borrower in accordance with
     its terms subject to applicable bankruptcy, insolvency, reorganization,
     moratorium or other laws affecting creditors' rights generally and subject
     to general principles of equity, regardless of whether considered in a
     proceeding in equity or at law.

          (b)  On and as of the Amendment Effective Date, the representations
     and warranties set forth in Article III of the Credit Agreement are true
     and correct in all material respects with the same effect as if made on the
     Amendment Effective Date, except (i) to the extent such representations and
     warranties expressly relate to an earlier date and (ii) representations and
     warranties that are not true and correct as a result of the failure to pay
     interest due on April 15, 2002 with respect to the Subordinated Refinancing
     Debt.

          (c)  At the time of and immediately after giving effect to this
     Amendment, no Default has occurred and is continuing other than the Default
     resulting from the failure to pay interest due on April 15, 2002 with
     respect to the Subordinated Refinancing Debt.

          SECTION 5. Conditions to Effectiveness. This Amendment shall become
effective as of the first date on or after May 10, 2002, that each of the
following conditions is satisfied (such date, the "Amendment Effective Date"):

          (a)  The Administrative Agent shall have received counterparts of this
     Amendment that, when taken together, bear the signatures of the Parent
     Borrower and the Required Lenders.

          (b)  The representations and warranties set forth in Section 4 of this
     Amendment shall be true and correct.

          (c)  The Administrative Agent shall have received (i) for the account
     of each Lender that executes and delivers a counterpart of this Amendment
     on or before May 13, 2002 an Amendment Fee equal to 0.25% of the sum of its
     Domestic Revolving Commitment and its outstanding Term Loans and (ii)
     payment of all out-of-pocket expenses (including fees and disbursements of





<PAGE>
                                                                               6

   counsel for the Administrative Agent) required to be paid or reimbursed by
   the Parent Borrower under the Credit Agreement, to the extent invoices
   therefor have been presented to the Parent Borrower at least one Business Day
   prior to the Amendment Effective Date.

     (d) The Administrative Agent shall have received (i) copies of each of the
   agreement attached hereto as Exhibit A, B, C and D, respectively, which shall
   have been executed and delivered by the parties thereto, and (ii) legal
   opinions, in a form and from counsel reasonably satisfactory to it, with
   respect to the due authorization and enforceability of such agreements and
   the absence of conflicts with respect thereto.

     SECTION 6. Effect of Amendment. Except as expressly set forth herein, this
Amendment shall not by implication or otherwise limit, impair, constitute a
waiver of, or otherwise affect the rights or remedies of the Lenders under the
Credit Agreement or any other Loan Document, and shall not alter, modify, amend
or in any way affect any of the terms, conditions, obligations, covenants, or
agreements contained in the Credit Agreement or any other Loan Document, all of
which are ratified and affirmed in all respects and shall continue in full force
and effect. Nothing herein shall be deemed to entitle the Parent Borrower to a
consent to, or a waiver, amendment, modification or other change of, any of the
terms, conditions, obligations, covenants or agreements contained in the Credit
Agreement or any other Loan Document in similar or different circumstances. This
Amendment shall apply and be effective only with respect to the provisions of
the Credit Agreement set forth herein. This Amendment shall constitute a Loan
Document for all purposes under the Credit Agreement. Each of the Exhibits to
this Amendment shall constitute a Loan Document and each of the Investor, DH and
the DH Investors (as each such term is defined in the Exhibits hereto) shall
constitute a Loan Party for all purposes under Article VII of the Credit
Agreement.

     SECTION 7. APPLICABLE LAW. THIS AMENDMENT SHALL BE GOVERNED BY, AND
CONSTRUED IN ACCORDANCE WITH, THE LAWS OF THE STATE OF NEW YORK.

     SECTION 8. Counterparts. This Amendment may be executed in two or more
counterparts, each of which shall constitute an original but all of which when
taken together shall constitute but one agreement. Delivery of an executed
signature page to this Amendment by facsimile shall be effective as delivery of
a manually executed counterpart hereof.

     SECTION 9. Expenses. The Parent Borrower agrees to reimburse the
Administrative Agent for its out-of-pocket expenses in connection with this
Amendment, including the reasonable fees, charges and disbursements of Cravath,
Swaine & Moore, counsel for the Administrative Agent.


<PAGE>
                                                                               7

     IN WITNESS WHEREOF, the parties hereto have caused this Amendment to be
duly executed by their respective authorized officers as of the day and year
first written above.

                                   KNOWLES ELECTRONICS HOLDINGS, INC.,

                                     by
                                       ------------------------------------
                                       Name:
                                       Title:

                                   JPMORGAN CHASE BANK, individually and
                                   as Administrative Agent,

                                     by
                                       ------------------------------------
                                       Name:
                                       Title:

                                   MORGAN STANLEY SENIOR FUNDING,
                                   INC., individually and as Syndication Agent,

                                     by
                                       ------------------------------------
                                       Name:
                                       Title: