check for the drivers for ur soundcard whether they are working or u need installing them. shud be able to get the drivers from the utility disk that you would have received with the pc. It could be that the drivers are not compatible with vista then ur best shot would be to go to the manufacturer's website and download the compatible drivers mate. Hope that ll help you.
try to use a usb sound controller it works in 98, xp, and i don't know other OS haven't tried, it's plug and play don't need a driver, i agree with lasher maybe your sound driver is having some problem with vista, if you want to go back to xp you can do it but you'll need some software, i have one it's called disk editor.. try searching on the web for this kind of file.. hope this help..
Note: If your sound card is on-board (and it probably is), then downloading and installing Vista drivers for your motherboard (whole bunch) from your motherboard manufacturer's site, not NVIDIA or whatever chipset site, would solve your sound problem. Sound drivers may be downloadable separately.