attachSound won't work in main movie!

1

Long time no see!

I’m attaching a sound in an swf, but it
doesn’t work in my main movie!

firstSound = new Sound();
firstSound.attachSound("sound.mp3");
btn.onPress = function() {
    firstSound.start();
    this.startDrag(false, 160, 375, 580, 375);
};

Is there a simple explanation for this?

thanks…