Comment 2 for bug 474304

mimomox, this could be for a couple of reasons. First, could you please double-check to see if all output devices in the audio mixer are unmuted (not just turned up)? Sometimes one device being muted will prevent other unmuted devices from working.

If that doesn't work, try installing the linux-backports-modules-alsa-karmic-generic package and rebooting. Again, installation of this package may cause some devices to become muted (or new devices are set to mute by default), so they need to be unmuted after the first reboot.

Let me know how that works.