Recently I was having a consistent problem with the sound on my Dell Inspiron 1720, which is running on Ubuntu Hardy Heron. It would consistently stop working all together, and I had a terrible time finding the cause of the problem. If I viewed a flash video on the web, the sound would die afterwards. If I listened to streaming Internet radio, the sound would die. If I received an emial or IM the sound would die.
Finally it dawned on me, and I am not sure why or how. But a few weeks prior I had been toying with a Bluetooth stereo headset and had turned on the Audio service in the Bluetooth manager on the services tab.
Problem fixed: I simply turned off the Audio service in the Bluetooth manager, and all is working normally again. However, if I ever need the Audio service I suppose it will break my sound.