This had been driving me insane for a little while. It seemed like after upgrading to Flash Player 9 that Flash audio stopped working. I even posted a bug on it in the internal Adobe bug database.
whoops. It turns out that one of my programs switched the audio output bitrate to 96k and for some reason, Flash doesn’t like that (it seems like every other program I have handles it just fine).
Anyway, if you have that problem, check the above link.