Failing that install a pci soundcard. I have had a dell GX520 (Although it may actually have been the 620) do exactly the same thing, If I connected it up to my amp and ramped the speakers all the way up you could still hear sound coming out, but it was just so low as to me unusable. I never did figure out what happened. Threw in a £4 soundcard and all was back to working happily.