If you are having problems with ALSA the first thing you should try is to reload the alsa drivers. In a terminal type:
killall pulseaudio sudo alsa force-reload pulseaudio -D
This is a temporary fix for many people who have problems with sound fading out or strange messages from ALSA caused by buggy drivers. More permanent fixes can only come from driver updates.
Reinstalling ALSA
It is also possible something has gone really wrong with the ALSA drivers or there is a problem with some configuration file that got messed up with all that fooling around from above. You can try purging and reinstalling ALSA. (I recently had to do this after replacing my motherboard, the new on-board sound card was correctly detected but my existing pci sound card was not, weird…)
(1) Remove the ALSA packages
sudo apt-get --purge remove linux-sound-base alsa-base alsa-utils
(2) Reinstall the same packages
sudo apt-get install linux-sound-base alsa-base alsa-utils
Packages gdm and ubuntu-desktop are also removed in this process if you are using Gnome. They need to be reinstalled:
sudo apt-get install gdm ubuntu-desktop
If you are using xubuntu this will also happen to you
sudo apt-get install gdm xubuntu-desktop
(3) Reboot.