[monolithed]

Что-то типо такого на ActionScript:

var url:String = "song.mp3";  //путь к файлу
var request:URLRequest = new URLRequest(url);  
var s:Sound = new Sound();

s.addEventListener(Event.COMPLETE, completeHandler);  
s.load(request);
var song:SoundChannel = s.play();  
song.addEventListener(Event.SOUND_COMPLETE, soundCompleteHandler);  
var ba:ByteArray = new ByteArray();  

var gr:Sprite = new Sprite();  
gr.x = 20;  
gr.y = 200;  
addChild(gr);  

var time:Timer = new Timer(50);  
time.addEventListener(TimerEvent.TIMER, timerHandler);  
time.start();  
  
function completeHandler(event:Event):void {
  event.target.play();  
}  

function soundCompleteHandler(event:Event):void {
  time.stop();  
} 

function timerHandler(event:TimerEvent):void {
  SoundMixer.computeSpectrum(ba, true);
  var i:int;  	
  gr.graphics.clear();  	
  gr.graphics.lineStyle(0, 0xFF0000);  	
  gr.graphics.beginFill(0xFF0000);	  	
  gr.graphics.moveTo(0, 0);  	
  var wave:int = 2;  	
  for (i=0; i<512; i+=wave) {  		
    var t:Number = ba.readFloat();  		
    var n:Number = (t * 100);  		
    gr.graphics.drawRect(i, 0, wave, -n);  	
  }

Кстати вот пример
А вот тут подробности реализации