Code:
// external movie-
_global.sndtrk = new Sound(this);
sndtrk.attachSound("my_trk");
sndtrk.start(0, 999); // seconds offset, loops

// main movie
function init(){
if(!running){
loadMovieNum("external.swf" 1000);
running = true;
}
};

init();
very similiar to code i posted to your other thread