Try this: In your childmovie put this script.
PHP Code:
function loadMySound(){
   
_root.holder.createEmptyMovieClip("clipHolder",1);
   
mySound = new Sound("clipHolder");
   
mySound.onLoad = function(success){
      if(
success){
         
trace("loaded");
         
checkLoading();
      }else{
         
trace("not loaded");
      }
   }
   
mySound.loadSound("soundfile.mp3",false);
}

function 
checkLoading(){
   
this.onEnterFrame = function(){
      var 
lBytes mySound.getBytesLoaded();
      var 
tBytes mySound.getBytesTotal();
      var 
rBytes = (lBytes/(tBytes/100));
      
progressBar._xscale rBytes;
      if (
rBytes >= 100){
          
_root.goToAndPlay("frameName");
      }
   }

You can also try to put checkLoading(); after mySound.loadSound

Regarding the mp3player there are still some minor bugs. I will soon put the final version on.