In my post on getting Glass working for a Virtual Machine, I spent a little bit of time discussing the graphics card that we emulate, and the reasons why Glass isn't possible on that hardware.
There's a similar issue in Vista with the sound card that we emulate. As you may have noticed if you've installed Vista in VPC, there's no audio support. This is due to the audio card that we emulate - an ISA-based SB16 card. Vista has dropped support for ISA-based hardware all together, so our emulated card simply won't work. Unfortunately, there's no way around it.
This will be addressed by emulating a different kind of sound card in a future version of Virtual PC.