Not such an easy diagnostic then :-( It is strange that changing
video would throw the sound, and even more so that it's just the
microphone side. If you have a surround sound type audio ( presumably
this is on-board too ? ) then try plugging the microphone into
different sockets. I have seen sockets mapped incorrectly.

Other than that, try different hardware selections in audio properties
to see if one of those works. Depending on the audio chip you may have
a number of options or very few.

Another thing to try is a live CD boot with Ubuntu and see if the
sound works properly in there.

What was the problem with the on-board video ?