This is why
it's usually start the sound file in a frame prior to the one you're now in. That is:
Frame 1: Begin sound.
Frame 2: Begin movie.
...
Frame end: gotoAndPlay(2);
That way, you don't get overlapping sounds. Alternatively, you can set a boolean value in that first frame, and in the second frame play the sound only if the boolean is true. Then set it false, and at the end of your movie, flip back to frame 2.