I've been using Ubuntu for some time, and I've started having this problem with my sound. I think it must have something to do with GNOME because it doesn't occur under KDE. I stick with GNOME anyway because KDE doesn't agree with me.
Anyway, I'm not sure what the cause is, but I usually notice when I'm trying to watch a youtube video. The video will play normally, but there will be no sound. When I try to close the window, my browser, firefox, freezes up for a little while and the next time or two I try to load a page after that, it freezes up again. It seems to happen in Chrome too, so I don't think it's the browser. Eventually I can go about my business as normal. This usually causes my flash-player to crash, however.
After that, none of my other sound-based applications will work properly. For example, my music player will open a file, but won't actually play it, if that makes sense. Like, the progress bar won't move, and there's no audio, and so on.
Alsa reports no problem, and the applet insists that I have sound. But clicking on "test speakers" doesn't give me anything except a smiley-face. I have to reboot the computer to get my sound back, but it will eventually crash again.