Comment 21 for bug 639846

I believe that the root cause is the hardware microphone + codec generates antiphase (opposite polarity) signals on the left and right channels. If these channels are added together, as happens when the stereo is converted to mono for playback on internal speaker, or in some voip apps, the result is a zero signal.

This is also being discussed here: http://comments.gmane.org/gmane.linux.alsa.devel/79489

Steps to confirm this:
* Install audacity for viewing audio, alsa-utils for mixer "sudo aptitude install audacity alsa-utils"
* Run "alsamixer" in terminal
* press F4 to get capture controls. Select each in turn using left and right arrows, raise to maximum using PgUp.
* Run Sound Recorder app. Select "CD quality - Lossy (.ogg type)" to record a few seconds of speech. Save the file as eg "test.ogg" (can also do with audacity, but more complex instructions required...)
* Run "audacity test.ogg", to see the waveform. See left and right are (vertical) mirror images - may have to zoom in.