If you have installed a sound driver, uninstall it and let Windows install the generic, basic function, HD Audio Codec (if it can) and then see how it works.
Go to the Device Manager, RIGHT click on the entry for the AC97 sound to highlight it and then LEFT click "Uninstall". Also Check to the box to also uninstall the drivers. Then restart the PC and when Windows starts Windows should detect the sound and install a driver. No guarantee this will work, but its worth a try.