I have a clean 12.04 install.
Imported my mp3's (from a windows share) into Rhythmbox and although they do appear to be playing no sounds is to be heard. Playing a radio-stream works just fine. Playing the same MP3 in Spotify works fine too.
I have looked at the mixer settings (but as radio is playing that does not seem to be the issue). Cannot find a reason why there is no sound...