Have you recently increased the ram? Some 32bit machines will lose sound if more than 3 gig is installed, as the bios allocates ram to the sound that cant be accessed by windows. If this isnt the case , and you find no answers here, you can buy a usb sound dongle on amazon for £10 that works fine for general use.
I just gave mine away. .
Dont buy the ones that cost 70 odd pence as they dont work AT ALL!