How to play next song

Question

I am trying to build a simple player inside my swf file. The code is a bit messy but it seems to work.
I´m using a soundChannel for Playback..
I´m trying to figure out how to have the player play the next song once the previous one has reached its end.
Do I add some sort of EventListener to the soundChannel?
Thanks for helping
import fl.transitions.*;
import fl.transitions.easing.*;
prevBt.buttonMode=true;
playBt.buttonMode=true;
stopBt.buttonMode=true;
nextBt.buttonMode=true;
pauseBt.buttonMode=true;
pauseBt.visible=false;
var xmlLoader:URLLoader= new URLLoader;
var temasURL:URLRequest= new URLRequest ("temas.xml");
xmlLoader.addEventListener(Event.COMPLETE, xmlLoaded)
xmlLoader.load(temasURL);
var temasXML:XML = new XML();
temasXML.ignoreWhitespace=true;
function xmlLoaded(e:Event):void{
      temasXML =XML(xmlLoader.data);
      var temaNum:Number=0;
      var tema:String = temasXML.tema[temaNum].mp3;
      var musica:Sound = new Sound(new URLRequest("audio/"+ tema))
      var sc:SoundChannel;
      var isPlaying:Boolean = false;
      var pos:Number=0;
      stopBt.addEventListener(MouseEvent.CLICK, parar_musica);
      function parar_musica(e:MouseEvent):void{
            sc.stop();
            isPlaying=false;
            playBt.visible=true;
            pos=0;
      playBt.addEventListener(MouseEvent.CLICK, tocar_musica);
      function tocar_musica(e:MouseEvent):void{
            pauseBt.visible=true;
            playBt.visible=false;
            if(!isPlaying){
                 sc= musica.play(pos);
                 isPlaying=true;
                 playerTx.text=temasXML.tema[temaNum].legenda;
     pauseBt.addEventListener(MouseEvent.CLICK, pausa_musica);
     function pausa_musica(e:MouseEvent):void{
           if (isPlaying){
               pauseBt.visible=false;
               playBt.visible=true;
               pos=sc.position;
               sc.stop();
               isPlaying=false;
     nextBt.addEventListener(MouseEvent.CLICK,seguinte);
     function seguinte(e:MouseEvent):void{
            sc.stop();
            temaNum +=1;
            musica= new Sound(new URLRequest("audio/"+ (temasXML.tema[temaNum].mp3)))
            sc= musica.play();
            pauseBt.visible=true;
            playBt.visible=false;
            isPlaying=true;
            playerTx.text=temasXML.tema[temaNum].legenda;
     prevBt.addEventListener(MouseEvent.CLICK,anterior);
     function anterior(e:MouseEvent):void{
            sc.stop();
            temaNum -=1;
            musica= new Sound(new URLRequest("audio/"+ (temasXML.tema[temaNum].mp3)))
            sc= musica.play();
            pauseBt.visible=true;
            playBt.visible=false;
            isPlaying=true;
            playerTx.text=temasXML.tema[temaNum].legenda;

Answer 1

if I declare:
     var musica:Sound;
then add the EL:
      musica.addEventListener(Event.COMPLETE, song_ended)
      function song_ended(e:Event):void{
            trace("finished");
            pauseBt.visible=false;
            playBt.visible=true;
            isPlaying=false;
and finally load the sound into the variable:
      musica= new Sound(new URLRequest("audio/"+ (temasXML.tema[temaNum].mp3)))
I get this error message:
     TypeError: Error #1009: Der Zugriff auf eine Eigenschaft oder eine Methode eines null-Objektverweises ist nicht möglich.
      at index1_fla::player_33/xmlLoaded()
      at flash.events::EventDispatcher/dispatchEventFunction()
      at flash.events::EventDispatcher/dispatchEvent()
      at flash.net::URLLoader/onComplete()
     (sorry about the untranslated text, I bought this laptop in Germany)
which leads me to think that the COMPLETE event in the Sound Class is related to the loading of the sound rather then the playing of the sound (correct me if I´m wrong).
On the other hand,
       var musica:Sound = new Sound(new URLRequest("audio/"+ (temasXML.tema[temaNum].mp3)))
      musica.addEventListener(Event.COMPLETE, song_ended)
      function song_ended(e:Event):void{
            trace("finished");
            pauseBt.visible=false;
            playBt.visible=true;
            isPlaying=false;
doesn´t work either. As a matter of fact, after I´ve written:
     var musica:Sound = new Sound(new URLRequest("audio/"+ (temasXML.tema[temaNum].mp3)))
it´s as if I can no longer add an Ev Listener (it´s not there on the list of options anymore)