When installing ubuntu 10.10 on an oldish laptop, the microphone was failing to pick up any sound. I first noticed this in skype, but going to sound preferences, input showed the same thing.

The issue was pretty obvious to see, there were no input connectors (showed up as software input), but much harder to work out the fix.

It turns out that a bug caused alsa-base.conf to be misconfigured preventing it from detecting the correct input devices for the sound card (on board intel).

To fix, the following line needs to be added to /etc/modprobe.d/alsa-base.conf

options snd-hda-intel model=3stack

Quick restart later, and all was good.