The problem persists with jaunty. The distortion is gone with pulseaudio, but still no sound without the workaround also mentioned above:
amixer cset iface=MIXER,name='IEC958 Playback Default'
This looks more and more like an alsa bug even though the output worked perfectly without tweaking the settings. I haven't tested audio without the manual workaround and without pulseaudio recently, i'll try and get that done during the weekend.