Well think about it. If your audio file is 2 seconds long and you change the fps of your movie from something like lets say 10 to 20 fps, then you are making twice as many frames play in any one given time. The audio is 2 seconds long and will always be 2 seconds long, so changing th fps changes its relation to flash’s timeline. Before, at 10 fps, 20 frames were needed to play to match the songs length. at 20 fps, it takes 40 frames to match the songs length, and you’ll see this reflected in the timeline with the stretching of the audio waveform.