My wife and I were rearranging the house this past weekend.  After I moved her computer I booted it up to make sure everything was ok and no sound.    Sound had always worked before, granted she didn't use it very often, in fact, often she mutes here volume control.  So I start looking at things it acts like it is playing sounds, mplayer and rhythm box are acting like they are playing songs but I get no sound out of the speakers.  Mute is off, volume is up but no sound.  I try to fire up alsamixer and I get an error message.  Alsa can not find the sound card.  OK, what gives how could Ubuntu lose the sound card.  It found it for the past year and it can't find it today.  I remember awhile  back there was a kernel upgrade from 2.6.28-17-generic to 2.6.28-18-generic  so I boot it using 2.6.28-17-generic.  The machine boots no sound but the volume was muted.  I turn off mute and sounds plays.  Great the old kernel works why doesn't the new kernel.  I decide to reboot using the newer kernel and now it plays sound.

For some reason since the sound was muted when the kernel upgrade was done when the machine was rebooted with the new kernel could not find the sound card.  But as soon as I booted using the old kernel, un-muted the sound, and rebooted into the new kernel the sound  card was found.  Don't ask me why I don't know why.  All I know is don't mute your sound if you  are upgrading your kernel.