I was suffering a long-standing problem getting sound to work on Flash Videos. Sometimes it would work, sometimes it wouldn't. When it did work then I couldn't play videos using other software - Mplayer, for instance. When I could get sound in Mplayer videos, then I couldn't get it in Flash. When editing sound files with Audacity, sometimes I would get sound, sometimes I wouldn't.
This state of affairs went on for a while until I discovered this old posting: http://ubuntuforums.org/showthread.php?t=732520.
By following the suggestions in the 5th and 7th posts on this page, and by following the instructions on setting up PulseAudio at https://wiki.ubuntu.com/PulseAudio, I now have no problems getting sound in any app.
I hope this proves useful for others!