Audio stopped working suddenly in 12.04

This happened to me too. I think the primary reason is the pulseaudio daemon isn't killed properly.

First run pulseaudio --kill then pulseaudio --start.

This fixed the problem for me.

There are two potential fixes, here are the two methods I tried (I'm not sure which of the two fixed the issue.)

Fix 1

I plugged in some headphones and killed pulseaudio:

pulseaudio --kill

then I started it again:

pulseaudio --start

This seemed to fix the issue from then on.

Using pavucontrol instead:

sudo apt-get install pavucontrol
pavucontrol