YouTube stopped working a while back on my Quad G5. Videos played, but with no sound. All other forms of video worked fine, and the exact same video would work if I downloaded it manually and ran it from a different FLV player.
I reinstalled Flash, I wiped caches to make sure I didn’t have an old version of the YouTube flash player, grabbed the latest version of Firefox and tried it there, etc, etc. No change. It wasn’t a big problem, since I usually don’t surf from that machine, but I finally spent a few minutes chanting Google incantations and found the answer:
Run the Audio MIDI Setup utility and change the Audio Output Format back to 44100 Hz