How does it behave when booted to OS X?
Two things that come to mind. One, there's probably a forum somewhere where this and other bootcamp issues are discussed - there may be a good one on apple.com.
Two, I wonder if you can find a more full featured driver for that sound controller. Sigmatel is a pretty common on board audio solution and I see it a lot when I reinstall XP on a variety of PCs. I don't know if Apple's version of that controller would be compatible with a generic or other manufacturer's driver. Might be worth a shot.