[FMX] custom Sound class not 'inheriting' onLoad

Hi all,
i’m new to kirupa. have read the brilliant OOP for actionscript 1 tutorial from cover to cover and it has helped me enormously. thank you!
My problem is with a class i wrote called frameSound which extends the Sound object. Well that was the plan but… it doesn’t seem to react to the onLoad event. Here’s the code:

//custom frameSound class constructor. 
  frameSound = function(){
      this.audioFrameLoaded = false;
      this.audioFramePlaying = false;
  //make frameSound subclass of Sound obj.
  frameSound.prototype = new Sound();
  //initialise frameSound.count
  frameSound.count = 0;
  //callback funcion when frameSound loaded.
  frameSound.prototype.onLoad = function(success){
      if (success) {
          this.sound_err = false;
      } else {
          this.sound_err = true;
          //dbug = "audio load error";
  frameSound.prototype.preloadSound = function (url, isStreaming) {
    //set off preloader interval stuff
    // Load the Sound file.
    this.loadSound(url, isStreaming);

When i used the above code to load an mp3 into a blank movie clip ‘frameSound’ object i get ‘undefined’ values for sound_err.
when i canged all this (and a few other methods added with frameSound.prototype… ) back to just being Sound.prototype functions it worked fine. But i’d like to keep the frameSound class seperate.

Any help would be much appreciated,