Ok, my current audio setup is as follows:
Onboard audio is a SigmaTel C-Major Audio device with driver version
5.10.4469.0 from SigmaTel.
USB audio is provided by a C-Media device running XEAR 3D, EAX 2.0 and
driver version 5.12.01.0042.7 (62.2).
Both drivers appear to be up to date and the devices are more or less
functional. I have two issues. First, when using the headphone jack on the
SigmaTel device, I have no output. Second, when I use 5.1 speakers with the
C-Media device, Vista's Speaker Setup wizard only registers them as stereo,
even though the C-Media program registers them as 5.1. Has anyone else had
either of these issues before? Can I still submit bug reports this late in
the game?

I should probably also note that this issue occurs under Vista RC2 build
5744. I have no surround sound output (steroe only) in WMP11 or Zoom Player
Professional (my video/dvd software of choice).