Just curious if something like gnome-alsamixer will work for you to let you see if something is muted? The alsamixer issue seems related to https://bugs.edge.launchpad.net/ubuntu/+source/alsa-driver/+bug/192382 which was fixed in alsa-source but hasn't yet been pulled into linux-ubuntu-modules (see comment from Daniel https://bugs.edge.launchpad.net/ubuntu/+source/alsa-driver/+bug/192382/comments/13)
I also notice you're running the 2.6.24-12 kernel. Care to install the latest updates to see if that helps. Thanks.