I'm not 100% sure on how this works in Flash, but the classes should be the same:
var url:URLRequest = new URLRequest("sound.mp3");
var snd:Sound = new Sound(url);
The Sound.play method takes three arguments. The first two are 1) start time and 2) loops. If you want to have it loop indefinitely you could create an event listener for when the sound stops and call a function to play the sound again.