Lately I had some trouble with sound on two Ubuntu 9.04 machines. After waking from hibernation sound would not work, or only work for a short time. After restarting alsa (/etc/init.d/alsa-utils restart) and pulseaudio (/etc/init.d/pulseaudio restart) I checked all volume settings with alsamixer, but nothing seemed out of the ordinary. After some more fiddling I found out that simply starting pulseaudio from the command line was enough to get my sound back again.