after installation of 12.10, only the left speaker produces sound (on 11.04, it was both)

I went through the recommendations of all the threads which I found on ubuntuforums but - no, it doesn't work - problem seems to be
"Playback channels: Mono"

What can I do ?

the information about my system is here: http://www.alsa-project.org/db/?f=8d...d4ec777e91546e


Thank you