I was recently having a problem with .mov files, whereby I'd get about .3 seconds of sound for every 3 seconds of movie. I upgraded to 6.3 and I think that fixed it.
It was really bad because I couldn't listen to Apple's own commercials.
It's the QuickTime 6.4 Alpha version that's having problems. Just wait for it to evolve or boot back into Jaguar for clean QT playback... And try VLC, it might help.
now that im back on spitty27 --- also quicktime embedded files in safari seem to play fine...avi, mpg, mpeg, mov, mp3. BUT safari randomly quits once you get towards the end of the segment.