Because thats what happened to me earlier..
I was “torturing” totem by runnin several instances and whatnot, when all sound failed and when it came back, it was all robot like. Metalic. A solution would have been to reboot bu I didnt feel like restarting all those programs when ultimately merely one service needed to be restarted. Ubuntu runs on PulseAudio these days, so all I needed to do was reboot the Pulseaudio daemon! (check the link, pulseaudio is Really awesome!!)
Pulseaudio runs user level so something like
sudo /etc/init.d/pulseaudio restart
wouldnt help (If you do run this, it will tell you pulseaudio runs on user level)
To solve your problem you ll need to find out what the pulseaudio pid is (marked red here)
ps -A | grep pulse 1886 ? 00:22:00 pulseaudio
then kill that pid
and if you want proof; check for success 😉
gert@Benedict:~$ ps -A | grep pulse
Then just restart totem or whatever and it ‘ll work again (the system will want to output audio and restart the pulseaudio daemon)
Hooray! Decent sound, No reboot!