Boot with the 12.04 Live CD and see if you get sound there, if not
then there's possibly a problem with the sound hardware but worth
trying a 10.04 Live CD to verify whether it's specific to 12.04.

Did you used to have sound on it ?

Might be work rolling back to a previous kernel too in case it's a
Kernel update that's screwed up. Hit [ESC] on boot to get the menu to
select alternate kernels.