Yes Miguel the exact smae thing happens to me when running Quicktime for prolongued periods, the playhead keeps moving but the audio/visual freezes. I have a feeling that OSX has a very long way to go .
Try the keepsoundawake app, it really works 95% of the time, according to its author the...
Yes there is a true problem with the sound. I use a PB G4 550 and the sound goes off even with Xounds installed. It will go on if I close the lid and bring it back from sleep. I got a free app called KeepSoundAwake that someone wrote to solve the problem and it seems to work fine...