I'm gonna assume you have xp. Under control panel, click on system, hardware, device Manager. Is there a yellow or red question mark/exclamation mark beside anything?
If so, you may need to install drivers for the sound card/onboard.
Also, in the bios, the onboard sound could have been disabled. Reboot your computer and press the 'Delete' key.
Search through the bios for the onboard sound, I can't remember what its listed under. If its disabled, enable it, save and exit.
When it starts windows should detect it and either install or ask fo rthe proper drivers.
Or I could be totally wrong
Good Luck