I was listening to music in Rhythmbox and paused it so I could watch a music video on YouTube. Because Rhythmbox had a pipe still open, I couldn't get sound in Flash. I've heard of people having problems with 2 outputs, but never on my laptop. So after talking to Crimsun for a bit on #ubuntu+1, I learned that if you use PulseAudio as your audio backend (which Hardy does by default), you need to have libflashsupport installed. My Flash came from Adobe.com, so the library wasn't automatically installed. If you're having trouble getting sound out of your speakers while a Firefox session that has used Flash is still running, try installing that library.