It might be that the email privacy litigation of the next century will involve corporation's rights to insist on a back door key to the employees encrypted email. The legal is even harder to understand than the social. Imagine a continuum between absolute employee privacy rights on the one end and absolute employer's rights on the other. Taken in this context, the email question may be merely the latest incarnation of the ages-old conflict between worker's rights and those of the corporation. On this account, WAV or AU email attachments are not private where the same audio information would be private if transmitted by phone.
Email is more persistent in the corporation while telephony is more ephemeral.
Isn't most email intended to be as temporary as telephony. Who wants to take the time to type a paragraph about the bosses' vasectomy anyway. Further, voice mail can be as persistent as email. An email box is like your desk drawer or company locker where there is no expectation of privacy. If this doesn't set up an adversarial relationship between employer and employee, nothing will.