stopping a sound file, Please help!!!!

Hi i have a program that allows me to press a key and it plays a sound here is the code:

stage.addEventListener(KeyboardEvent.KEY_DOWN, myKeyDown);
function myKeyDown(e:KeyboardEvent):void
{
if (e.keyCode == 65 ) // A
{
var MySound:Sound = new Soundfile();
MySound.play();
}
}

This works fine but if i press the a key again before the sound has finished i want it to stop the sound and start it again. I thought i could do it like this:

function myKeyDown(e:KeyboardEvent):void
{
if (e.keyCode == 65 ) // A
{

var MySound:Sound = new Soundfile();
MySound.stop();
MySound.play();
}
}

by stopping the sound before i start it that way if it is playing and the key is hit it will stop and start again, i get this error though:

Error #1006: stop is not a function.

please please can someone help with this. oh iam using cs4 as3

for external sound...

Code:

---

stage.addEventListener(KeyboardEvent.KEY_DOWN, myKeyDown);
var MySound:Sound;
var channel:SoundChannel;

function myKeyDown(e:KeyboardEvent):void {
        if (e.keyCode==65) {
                // 65 is a
                if (! MySound) {
                        MySound = new Sound();
                        MySound.load(new URLRequest("song.mp3"));
                } else {
                        channel.stop();
                }
                channel=MySound.play();
        }
}

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