How to fade in-out groups of audio sources?

Hi, I have this script for play groups of audio sources when the gameObject "audio zone" is triggered. Can somebody help me for start this group of audio clips with a fade in and stop them with a fade out when the gameObject object "audio zone" is triggered?

 #pragma strict
  
 var audioSources;
  
 function Start(){
     audioSources = gameObject.GetComponentsInChildren(AudioSource);
 }
 function OnTriggerExit(){
     Debug.Log("stopping sounds");
     Debug.Log(this);
     var self = this;
     for (var source:AudioSource in audioSources) {
         Debug.Log(source);
         source.Stop();
     }
 }
  
 function OnTriggerEnter(){
     Debug.Log("starting sounds");
     Debug.Log(this);
     var self = this;
     for (var source:AudioSource in audioSources) {
         Debug.Log(source);
         source.Play();
     }
 }