Update: In Intrepid Ibex this package is now called flashplugin-nonfree-extrasound.
as some of you may have already recognized, in Ubuntu Hardy, normale sound applications and Flash in Firefox don’t play well. For example: if you first start Rhythmbox, hear some music and later run Firefox to watch some Youtube it plays without sound.
This problem is known and there is a solution! Just install libflashsupport.
Cite from the description:
Due to various bugs in the Flash 9 plugin sound output of Flash 9 through the pulseaudio soundserver doesn’t work properly. This library adds a clutch to make Flash 9 sound output in pulseaudio possible.
For further information see http://www.pulseaudio.org/ticket/43 and http://www.pulseaudio.org/wiki/FlashPlayer9Solution