On one of my laptops (Medion MD96290) I noticed that the sound wasn't working any more under Ubuntu 8.04. A while ago, I managed to get sound going in Ubuntu7.10 (http://forum.ubuntu-fr.org/viewtopic.php?pid=1290804)
After some research (http://ubuntuforums.org/archive/index.php/t-765671.html) I added the following line to /etc/modprobe.d/alsa_base:
options snd-hda-intel model=6stack-dig
This didn't help. In the same forum, they suggest to enable surround. In Edit/Preferences you can display the Surround slider.
And the magic happened: the sound workz :-). Setting up sound can be quite complex in Ubuntu...
update 15/11/2008: I had to repeat the same actions after upgrading to Ubuntu 8.10: add line to alsa-base file and enable the surround slider.