After much googling I found the answer to getting the internal mic to work.
First of all, when I type alsamixer -c0 my sound card is called HDA Intel PCH and the chip is Intel CougarPoint HDMI. If yours isn't this it might not work.
As root, edit (create if it doesn't exist) /etc/modprobe.d/sound.conf
Put in there: options snd-hda-intel model=asus
Reboot! It should work. Make sure the mic isn't muted if it didn't.