I am not really sure how I fixed it on Vista Pro 32 but I believe it started to work when all Windows updates were downloaded and installed. You should also go download the latest Realtek HD Audio Driver from their website.
I am running Window 7 Pro 64 now (it is much better than Vista! I would only buy the Win 7 Pro if you plannig ot upgrade bc Win 7 Home is lacking while Win 7 Ultimate is excessive unless you need to run it in different languages.) and no problems what so ever!