Question: The group that were the singing voices of the Be Sharps on The Simpson's episode "Homer's Barbershop Quartet" have performed at Walt Disney World since what year?

Context articles: Vocal Spectrum
The Dapper Dans
SALT (quartet)
Forefront (quartet)
Realtime (quartet)
Barbershop in Germany
Walt Disney World Dolphin
Four Voices
Homer's Barbershop Quartet
Metropolis (barbershop quartet)

Vocal Spectrum is a barbershop quartet from St. Charles, Missouri.
 In 2004, Vocal Spectrum won the Barbershop Harmony Society's International Collegiate Quartet Contest, and on July 8, 2006, they became International Champions, winning the society's International Quartet Contest.
 A distinctive factor of the quartet is tenor Tim Waurick's ability to sustain notes for upwards of 30 seconds, and the tenor's and lead's incredibly high note range, featured in many of the group's recordings and live shows.
The Dapper Dans are a barbershop quartet that performs at Disneyland in Anaheim, California (since 1959), at the Magic Kingdom at the Walt Disney World Resort (since 1971), and at Hong Kong Disneyland in Lantau Island, Hong Kong (from 2005 to 2008).
 A quartet (from the UK) also performed at Disneyland Paris from the opening in 1992 until 1995 and were known as the Mainstreet Quartet.
SALT is a Swedish barbershop quartet that won the Sweet Adelines International Quartet Championship for 2007 in Las Vegas, Nevada in October 2006.
 Sweet Adelines, "one of the world's largest singing organizations for women", has members over five continents who belong to more than 1200 quartets.
 The quartet placed third at the international contest in 2003 (Phoenix) and 2004 (Indianapolis), and earned a second-place finish in 2005's contest in Detroit.
 SALT's November 2008 feature performance in Anaheim, California, was noted by the Vasa Order of America.
 At the time of winning the Sweet Adelines competition, SALT included tenor Anna Öhman, also a music teacher; lead Annika Andersson, a manager at a social security agency; baritone Anna-Stina Gerdin, an elementary school teacher; and bass Susanna Berndts, a speech therapist.
 Karin Sjöblom later replaced Gerdin as baritone.
 The quartet announced its retirement on Facebook in June 2012 "after singing together for 13 years".
Forefront is the barbershop quartet that won the International Quartet Championship for 2016 at the Barbershop Harmony Society's annual international convention, in Nashville, Tennessee.
 The quartet had placed second in the 2014 and 2015 international contests, after competing at the international level annually since 2010.
 Forefront formed in August 2009.
Realtime is a barbershop quartet that won the Barbershop Harmony Society's International Quartet Championship in 2005.
 They finished seventh-place the previous year (the first ever top-ten finish for a western-Canadian quartet).
 Lead singer John Newell is the first Australian-born international champion.
 Baritone Mark Metzger and bass Tom Metzger are the first champions who are both Canadian-born and Canadian citizens.
 Tenor Tim Broersma is from Lynden, Washington.
 In 2008, Doug Broersma became the lead singer when Newell retired for family reasons.
 The quartet was succeeded by Via Voice when Tim Broersma retired in 2011, also for family reasons.
Barbershop in Germany (abbreviated BinG or BinG!)
 is the association for barbershop music in Germany.
 It was founded in 1991 by the members of a small German barbershop fan group, and the organization is an official affiliate of the US-based Barbershop Harmony Society.
 Any representative of a German barbershop quartet or chorus can become a member of BinG.
The Walt Disney World Dolphin is a resort hotel designed by architect Michael Graves located between Epcot and Disney's Hollywood Studios in the Walt Disney World Resort in Bay Lake, Florida, next to Disney's BoardWalk Resort area.
 It opened on June 1, 1990 and is joined to its sister hotel, the Walt Disney World Swan (also designed by Graves) by a palm-tree lined covered walkway crossing a lagoon.
 The Walt Disney World Swan and Dolphin is a joint venture between the Walt Disney Company, Tishman Hotel Corporation, MetLife and Starwood Hotels and Resorts.
 The land the resort occupies is owned by the Walt Disney Company, while the buildings themselves are leased by Disney to the Tishman Hotel Corporation and MetLife but operated by Starwood Hotels & Resorts Worldwide under the Sheraton Hotels brand.
 The Walt Disney World Swan and Dolphin are a part of the Walt Disney Collection of resorts; because of this they are Disney branded and guests of the resort have access to special Disney benefits available to Disney Resort Hotel guests only.
Four Voices is a barbershop quartet based in Tennessee.
 After winning the SPEBSQSA Collegiate Barbershop Quartet Championship in 1996, Four Voices went on to become international champions in 2002.
"Homer's Barbershop Quartet" is the first episode of "The Simpsons"' fifth season.
 It originally aired on the Fox network in the United States on September 30, 1993.
 The episode was written by Jeff Martin and directed by Mark Kirkland.
 It features the Be Sharps, a barbershop quartet founded by Homer Simpson.
 The band's story roughly parallels that of The Beatles.
 George Harrison and David Crosby guest star as themselves, and The Dapper Dans provide the singing voices of the Be Sharps.
Metropolis is a barbershop quartet affiliated with the Barbershop Harmony Society.
 The group won five consecutive medals with the Barbershop Harmony Society at their annual International Convention.
 Metropolis has performed over 460 stage shows in the United States and in Finland, Russia, England, New Zealand, The Netherlands, Japan and Ireland.
 They have performed in over 190 cities around the world and over 40 states/provinces in the USA & Canada.