This is one of the many things that are plaguing Vista at the moment. There's two possible problems, as I see it. One is drivers that are not working right or haven't been released. Another may be compatibility of the hardware itself. Its the big reason that though I want to build a new machine, I'm waiting until after Vista RTM comes out, and then give allow some time to work the bugs out.
I expect there will be a lot of surprises when Vista finally does appear, and they won't be good ones. I can remember when Win95 first came out. Microsoft said that 99% of the Win 3.1 programs would work with it. Of the programs I had, 99% did not work, not even programs made by Microsoft. Then there were hardware problems. The computer shop that sold it to me couldn't fix it. They sent it back to the company twice and the company people couldn't fix it. Finally they gave me my money back and suggested I buy a new machine after 6 months, when things were stabilized.
Since you sound card works with XP 64 Pro, it shouldn't be a problem with the 64 bit driver, but something in Vista itself. Hopefilly, in a month or two, Microsoft will issue a patch or something to fix the problem. I suspect they will be issuing a lot of patches in the coming months.