I have a SoundMAX as well and one of the things I don't like about the Vista audio stack is that they now search the hardware on board (or something similar according to Larry Osterman) to give the supported output channels.  I have a 5.1 audio speaker setup that I have working on XP, but in Vista I can't output 5.1 channels, which renders those speakers useless.  Back on topic, I consider the Vista audio stack immature if not incomplete.

Edit: In Vista's defense, it COULD be that my drivers aren't properly reporting the audio channels it supports (or XP was allowing 5.1 channel output through stereo which is fine by me and my speakers), but I also used the drivers that Windows Update served me for the sound card.