Have you tried to reinstall audio drivers?
What are your temps?
I've been saying this a lot lately but running a livecd of linux to see if you have sound should tell you if this is a hardware problem or not.
If you are unfamiliar with a livecd, basically you burn an iso image to a CD and boot to the cd drive and you have a working copy of linux running on your system. When you are done testing, you remove the CD and reboot and you are back to your Windows Vista install.
I've been recommending Ubuntu lately and here is their download page with instructions.
https://help.ubuntu.com/community/LiveCD
Let us know how it goes.
Pauldo