Comment 37 for bug 271519

Craig Keogh above is spot-on!

I had the same problem, although not in Ubuntu, but openSUSE!! My audio was functioning correctly, but at a very low level / volume.

I used the gnome Volume Control's Preferences dialog to select the Front channel and raise to 100%, it was at 50%, which kind of verified the impression I had of my audio. Raising it to 100% restored my audio to normal.

In fact, if you go through all the channels in the list, you'll see that many of them are at 50% by default!

So, this appears to be something ALSA / GNOME related, not to Ubuntu.