Same problem here. When I load a website that has sound (such as YouTube), a little red light comes on (looking into the headphones port) and the volume controls all have a circle/slash thru them.
In the system preferences>>sound ------ the option for "internal speakers" does not appear (as it does on my other Macbook). The only choice shown is "digital out". So the system software thinks there are no internal speakers.
Is there an Apple tip sheet how to get the sound going again? Not interested in any hacks, workarounds, probes, paperclips, drivers. How does an Apple tech solve this problem on a Macbook Pro running OS X 10.6.8?