So...with that tutorial Fruitbeard posted, you can do something like this to loop the music:

PHP Code:
onFrame (3) {
  
mySound = new Sound();
  
mySound.loadSound("song1.mp3");
  
mySound.start(0999); 
mySound.onSoundComplete = function () { 
mySound.start(0999); 


I've not tested it so i can't tell 100% it will work, but it should. Just replace "onFrame (3)" with the number of the frame of the scene you want the sound to start playing. It will keep playing throught all the frames until the last frame. If you don't use a stop(); on the last frame, the movie probably will repeat from its first frame, so the sound will start again over the already playing sound... so if you come up to this situation, i will read the swish tutorial to help you out.
In as2, i would do something like:
PHP Code:
onFrame (3) {
if(
mySound.position != mySound.duration)
{
trace("it's playing");
};
else
{
trace("it's not playing");
  
mySound = new Sound();
  
mySound.loadSound("song1.mp3");
  
mySound.start(0999); 
mySound.onSoundComplete = function () { 
mySound.start(0999); 
}