Comment 3 for bug 965495

I have the same problem as well with the same version of Mumble and Ubuntu 12.04 on a Thinkpad X with Intel CougarPoint HDMI audio and my internal speakers and microphone. My internal microphone has two channels and presents itself as a stereo device in PulseAudio.

I found a workaround which uses PulseAudio's own echo canceller instead of Mumble's. For it to work, you must use PulseAudio for your sound input and output. From the console,
$ pactl load-module module-echo-cancel

This command will create another PulseAudio sink/source pair for echo cancellation, named "Echo-Cancel Source xxxx" and "Echo-Cancel Sink xxxx." Select these in Mumble for your sound input and output devices. Be sure to disable Mumble's echo cancellation. (Set "Echo" to "Disabled" in your Audio Output preferences.) This works much better than Mumble's own echo canceller on Precise.