Recently I installed Ubuntu Karmic (9.10) and everything worked fine until I made first upgrade. since then I had no sound. Solution is quite simple.
Btw. I assume that U have mediubuntu repositories installed.
sudo aptitude install alsa-firmware alsa-firmware-loaders linux-backports-modules-alsa-karmic-generic linux-backports-modules-‘uname-r’
Last step was to edit /etc/modprobe.d/alsa-base.conf and /etc/modules.
After restart, everything should be working fine.