Try the uninstall/reinstall trick. It fixes a lot of problems and MAY take care of yours. Go to the Device Manager, RIGHT click on the entry for your sound device to highlight it, then LEFT click "Uninstall" DO NOT check the box to uninstall the drivers. Restart the PC and when Windows starts it will detect and reinstall the sound.
To access the Device Manager, hold the Windows key down and press the Pause/Break key. When the panel comes up, the Device Manager will be in the upper left Task List.