First instinct would be to re-install ALSA but since 8.04 is out of
support there are no repositories live that you could do this through.
One of the big problems with continuing to run an older Ubuntu, fine
whilst everything is working but fixing it up when it goes wrong can
be a nightmare

When you boot into 8.04 do you have any older kernels listed ? If so
then you can try booting into an earlier one and see if sound works in
that.