After upgrading to Firefox 2 I had problems with the Flash plugin no longer producing sound when another audio application was running - a problem I'd not previously had with Firefox 1.5.
The fix was fairly simple. I had libflashplayer.so in my .mozilla/plugins directory. This was being used by Firefox 2 for the application/x-shockwave-flash MIME type in preference to Shockwave Flash v9.0 (flashplugin-nonfree package).
After removing libflashplayer.so and flashplayer.xpt from my .mozilla/plugins directory audio playback works perfectly in the Flash plugin in Firefox 2.