It looks like that this is a problem with the integrated speakers. Reinstalling drivers is unlikely to help if sound is fine using headphones or external speakers. Unfortunately integrated speakers tend to be pretty small and not very strong to save room on the notebook itself. If you aren't getting any sound at all or very low sound with the integrated speakers then you may wish to consider replacing them.
If you feel that the notebook is still under warranty and in the US I can process a service myself. Otherwise you check out the user' manual here for instructions on replacing the speakers. The part number for the speakers are R6JWX.