Well, now that more and more fanboys have upgraded to 10.7, the forums are full of it: sometimes it happens that sound stops working in MacOS Lion.
As it turns out, restarting coreaudiod is all it takes to get it working again:

$ launchctl list | grep audio
7159    -       com.apple.audio.coreaudiod

$ launchctl stop com.apple.audio.coreaudiod

$ sudo launchctl list | grep audio
7589    -       com.apple.audio.coreaudiod