Comment 8 for bug 412862

Somewhere internally the polarity of one of the input sources is reversed with respect to the other which is causing phase cancellation.

Using either the internal microphone or a mono external microphone connected to the socket the same signal is applied to left and right channels (mono mics are usually wired to both pins) so when both are set to exactly the same level total cancellation occurs.

This can be clearly seen if pavucontrol is installed, the mic level controls unlocked and one of them adjusted. As it approaches the same level as the other cancellation occurs then sound returns as it goes past it.

If a stereo source is applied to the external socket anything which is the same on both channels is cancelled leaving only the differences.

This seems to be happening with multiple Acer Aspire models but does not happen when running Windows so is unlikely to be a hardware issue.

My guess is that it is a driver or alsa problem but I have not had time to investigate further.

Sean.