12.04 Precise, Asus Eee 901, internal microphone not working
I've recently upgraded my aging Eee 901 from it's previous Oneiric installation to Precise, and at first glance, everything works smoothly, except this one thing. Extensive googling failed to produce a sensible answer so far.
Eee 901 has an internal microphone. I can see the volume control for it in console alsamixer, so it's there. If I turn the mute off on it, I can hear the feedback noise in the speakers. In short, it appears to be working correctly at the alsa level. It worked fine in Oneiric.
However, after the upgrade, Ubuntu insists that when the microphone jack is not inserted, I do not actually have input devices -- the input devices window is blank. There's basically nothing to tweak in the sound control panel anymore, so if there's a setting responsible for this behaviour, it's hidden well.
No amount of messing with pulseaudio through pavucontrol etc. has so far resulted in the appearance of a microphone input device in the sound control panel when no microphone is plugged into the external mic socket.
Where do I dig next?