For some time, there was no sound for Flash movies on my MacBook, and I couldn't find any way to fix it. Even tried the new Flash 9 beta with no success.
I finally found the solution on MacOSXhints: open GarageBand, play anything, and you're done. Flash sound is back.
Looking further in the hint's comments, it appears that some system updates initializes the sound system to wrong values, and that GarageBand, or any other application using the Midi subsystem corrects these values upon start.