Comment 42 for bug 986817

There was a bug with multichannel duplex where the number of output and input channels are different, which should be fixed by:

http://cgit.freedesktop.org/pulseaudio/pulseaudio/commit/src/modules/alsa?id=02dc6d84edde7af86e155b224b6aa98080ef2979

That fix has not been released into Ubuntu yet, though.