Hello,
Hold down your windows key and then press the pause/break key. This brings you directly into system info(minus all the clicks). Go to hardware tab, and then device manager. Look under sound.
Do you see any yellow exclamation marks? Yellow exclamation marks indicate faulty devices.
You could always try adding in an old sound card (they are usually cheap if they are used-- or better yet, borrow one from another computer to test) via a pci slot. You can then disable the integrated in the BIOS (or possibly on the motherboard via a jumper, but it's more likely to be in the BIOS. <-- This would be my next step.
Step by step instruction: http://www.infopackets.com/channels/en/windows/gazette/2004/20041228_install_sound_card_to_computer_with_onboard_integrated_sound.htm
Regards,
-Emily