Overall you'll save movie space if you just use Flash to stream the file on loop. This is the code that you'll need.
(s= arbitrary variable)
s = new Sound(); //initializes a new sound
s.loadsound("c:\yourpathhere\file.mp3", true); //loads sound into player, uses URL, the true/false value isStreaming tells the player if the sound is streaming
s.start; //starts the sound file
Of course, there is another way. If you would like to load the file into the library you can use a runtime variable to attach the sound and play it. You'll need to right click on the file you want to play in the library and give it a linkage identifier (in this case linkname).
s= new Sound();
s.attachSound("linkname");
s.start(0,2); //the values for the parameters are (offset, number of loops);
Hope either works. If not, repost and I'll supply what help I can.