Display microphone volume level in actionscript 3

The steps to design a circle, whose size increases as microphone volume increases in actionscript 3.0
1) Click Adobe Flash CS3 Professional:
2) File->New ->ActionScript 3
3)  In “layer 1, press F9 to import following actionscript
var myMic:Microphone = Microphone.getMicrophone();
Security.showSettings(SecurityPanel.MICROPHONE);
myMic.setLoopBack(true);
myMic.setUseEchoSuppression(true);

stage.addEventListener(Event.ENTER_FRAME, stage_EnterFrame);
var zw:Number = stage.stageWidth/2;
var zh:Number = stage.stageHeight/2.;
function stage_EnterFrame(e:Event)
{   
var num:Number = myMic.activityLevel*5;
graphics.clear();
graphics.beginFill(0x555555);
graphics.drawCircle(zw, zh, num);
}
4) Control + Test Movie (ctrl+enter) to test movie. Export movie to swf file.
5. The swf  is below: you can not see the circle until you turn on microphone and speak
(Reference: http://www.youtube.com/watch?v=IYmdkvLVRgc
http://www.gotoandlearn.com/
http://www.brendandawes.com/project/source/ )