Does the sound work when you test the movie in Flash?
If not (or even if it does, just in case), is the music layer extended the whole movie?
If yes and it still doesn't work, try deleting that music on the timeline, creating a new movie clip, putting the music in the blank keyframe in the movie clip and extending the frames in that clip until the song ends. Then put that MC on your main timeline, and extend the frames of that MC as long as you want the music to play.
That's assuming you have the music on its own layer, by itself, which you should.
Good luck.
