I'm sure you already checked the following, but just in case not...

1) Make sure you don't have the sound slider on minimum, and verify that you're not muting the thing.

2) IIRC, there is a coupling capacitor between the audio driver and the speaker. During your tracing, you undoubtedly encountered it. That capacitor has a habit of dying, so if you can't find anything wrong, just replace the cap and see what happens.