Flex sound auto stop at the end of music

Question

Tie this code to a button, if I repetitive press as fast as I can, an error exception would appear in the application. I found that it would not throw an error if I stop the sound when it completely fade out. However, I wish the fade out function could stop the sound individually, not stop o1_1 together.

private function p1_1():void{
s1_1 = new Sound();
s1_1.load(new URLRequest(a1_1));
o1_1=s1_1.play();
}

private function fade1_1():void{
TweenMax.to(o1_1, .35, {volume:0, onComplete:stop1_1});
}

    private function stop1_1():void {
//if I add, this code would only stop the sound altogether rather than individual. I wish to have each of the instance stop itself.
    o1_1.stop();
}