Assuring a single instance of sound

I have this code:

var mySound:Sound = new Sound();

var myChannel:SoundChannel = new SoundChannel();

var lastPosition:Number = 0;

mySound.load(new URLRequest("audio/audio1.mp3"));

pause_btn.addEventListener(MouseEvent.CLICK, onClickPause);

function onClickPause(e:MouseEvent):void{

    lastPosition = myChannel.position;

    myChannel.stop();

}

play_btn.addEventListener(MouseEvent.CLICK, onClickPlay);

function onClickPlay(e:MouseEvent):void{

    myChannel = mySound.play(lastPosition);

}

stop_btn.addEventListener(MouseEvent.CLICK, onClickStop);

function onClickStop(e:MouseEvent):void{

     SoundMixer.stopAll();

}

Everything works fine : When I click the play button it starts and resumes sund. But if  I accidently press the start buttun twice it brings on two simultaneos sounds. How can I make sure that only 1 instance of sound is playing?

Thanks for any help.