Google found some hints here:
But none of the hints would solve my problem exactly. The key seems to be the Audio MIDI Setting application. Open a shell and type:
open "/Applications/Utilities/Audio MIDI Setup"
Twiddling with the volumes at the bottom of the first pane and setting the output device correctly solved the problem for me. Others reported that they needed to (re-) set the sampling rate to 44100.
This is of course only a workaround. This Flash stuff is real crap as always. You always need the newest version to be able to do stuff you could easily do without it. And it prevents you from doing useful stuff which you could do without it, all for commercial reasons. It does not play fullsreen fluently on my 1.25 GHz iBook while this plays DVD with much higher bitrate and framerate fine, i.e. it does not use the hardware scaler. This crap sucks.
By the way: This is exactly the type of behavior I expect from Windows machines: Install something and something completely unrelated breaks. Apple: Please make this more robust!
So long ... happy youtubing! :-)