If you built this computer, the sound drivers are most likely on the CD that came with the motherboard. If you know what kind of audio chipset or audio card is installed, you can go to the manufacturer's web site and download the drivers.
Your best bet may be simply to do a system restore to a time when the sound was working.
Go to the pc manufactuer's (Dell, HP,...) web site and find your model computer. They will have drivers for you to download/install. I'm surprised the system restore didn't do the trick. It should roll drivers back.