No sound in Flash

[Walter Hurry]

FreeBSD 9 (x86_64).

Sorry if this is a FAQ, but I have googled assiduously and found nothing 
useful.

I have installed Flash, following the instructions in the handbook.
It works well, and the video element seems fine and smooth. But on (for 
example) YouTube, there is no audio at all.

This is despite the fact that .mp3s, .mp4s, .avis, .movs  etc. all play 
perfectly (in mplayer).

There is probably a simple solution, but I cannot find it.

Can anyone help?