Sounds like its either the Sound Drivers or the Sound Codecs. Try installing a Codec pack such as Klite
. Download the Standard Pack. Make sure you select the audio codecs during the installation. See if that helps.
For the drivers, update to the latest here
The order I would follow (assuming you've tested the speakers, with another device, and unplugged/replugged the speaker wires leading to the sound card):
1. Update drivers, test.
2. Download Codecs, test.
3. Install another Sound card (if you have one)
4. Reinstall Windows
5. It's the Motherboard. Contact your motherboard manufacturer for warranty details (RMA).