Loaded sound movie

Ok so I have a main movie file, and a seperate movie for the sound to keep the file sizes low.

The Sound movie works fine when you preview it, but when the sound movie
is loaded into the main movie, it loads but the sound doesnt play.

here is the code im using for the sound movie:

var song:Sound = new Sound();
play_btn.onRelease = function() {
stop_btn.onRelease = function() {

Any one have any ideas why its not playing? I cant figure it out.

Cheers, Simon.