I'm using kde 4.1 but not compiz. And, I'm scratching my head
I left all settings alone, waiting for some suggestions here. After a reboot, its all working again. Youtube works smoothly. I'm glad it's fixed, but it's frustrating not knowing what is going on. If it ever happens again, I don't have any tools (but a reboot) to try and get it fixed. It would be nice to be able to share a solution with others. Sorry.
I did look at audio settings, but didn't change any. Perhaps its the Heisenberg Principle. I changed things merely by looking at them.