codice:
var songTrack:Sound = new Sound();


function checkProgress (soundObj:Object):Void {
   var numBytesLoaded:Number = soundObj.getBytesLoaded();
   var numBytesTotal:Number = soundObj.getBytesTotal();
   var numPercentLoaded:Number = Math.floor(numBytesLoaded / numBytesTotal * 100);
  if (!isNaN(numPercentLoaded)) {
      
     /* Questa trace la vedi correttamente ??
     se SI allora forse non hai scritto il nome di istanza o variabile
     al tuo campo dinamico
     */
      trace(numPercentLoaded + "% loaded."); 
     campo_progresso.text= numPercentLoaded + "% loaded."
  }
};

songTrack.onLoad = function ():Void {
     trace("Complete");
    clearInterval(poll);
};

songTrack.loadSound("http://mp3_path", true);

var poll:Number = setInterval(checkProgress, 100, songTrack);