I've just upgraded from 11.10 to 12.04. 11.10 works fine, but in 12.04 it seems that everything is failing. When I play a video or music, sound (and video) is (faltering?), and YouTube does not work well.
I've tried lots of programs and installed codecs from medibuntu, but it seems there is a problem with the audio driver or even with kernel. I have hardware acceleration via nvidia proprietary driver, if it is relevant.