If you are on ubuntu hardy I feel your pain. It completely blows. Perhaps this will help, the result of many many painful hours combing the forums. It seems to do the trick for me, anyway. I programmed this as a bash shortcut so I wouldn't forget.

  I don't really know what the fix is when things crap out, I have just kind of tried things blindly.
  http://ubuntuforums.org/archive/index.php/t-968697.html

  (?) sudo alsa reload
  sudo alsa force-reload
    also: look for a dialog that pops up, possibly in task bar, if it asks if reload is ok, it is. beh.

  also related
    https://bugs.launchpad.net/ubuntu/+source/pulseaudio/+bug/294859
  also suggested:
    sudo killall pulseaudio
    sudo alsa force-reload

  also:
    rm /var/lib/alsa/asound.state (don't include the rm if googling for this post)
    sudo force alsa-reload
      http://74.125.77.132/search?q=cache:faaeSQtYS4cJ:pennsylvania.ubuntuforums.com/showthread.php%3Fp%3D6242032+gvfs+alsa&hl=en&ct=clnk&cd=5&gl=pl