Comment 6 for bug 1004819

Hmm, could you try the following:

1) Uninstall the dkms package so you have a clean precise kernel.
2) Add this row to /etc/modprobe.d/alsa-base.conf
options snd-hda-intel model=auto
3) reboot

Is this making internal mic work (without causing other changes/regressions for other outputs/inputs)?