I had a load of trouble with a on board Realtek HD sound chip a few years back,just stopped working for no apparent reason,bit a bit of googling and found it was not a uncommon problem,it required a download from microsoft of a HD bus driver.
Dunno why this suddenly happened but I downloaded the recommended file and suddenly I had sound again.
Try this website.
Download Microsoft UAA Bus Driver for High Definition Audio Driver for Windows NT / Windows 2K / Windows XP - Softpedia