Thanks for your reply. But I guess i wasn't to specific about my problem. The rear speakers just don't work, no sound, nothing, null...
It looks like they are muted or sth and I don't know how to unmute them and it would be nice to control the volume too directly from the mixer.
I looked in the specifications for ALSA drivers and they have the support for rear and front speakers.
Could I somehow unmute them with amixer? And how?