Audio fade after entering trigger?

I want to fade out one audio clip and fade in a different audio clip when entering a trigger. The code seems to work, when i replace function OnTriggerEnter with function Update. The audio clips are fading in and out, when starting the game. Any ideas? Thanks in advance.

var track1 : AudioClip;

var track2 : AudioClip;

function Start(){

audio.clip = track1;

audio.Play();

}

var audio1Volume : float = 1.0;

var audio2Volume : float = 0.0;

var track2Playing : boolean = false;

function OnTriggerEnter() { //replacing with "function Update" works and the audio clips are fading in and out, when starting the game

   fadeOut();
 
 if (audio1Volume <= 0.4) {
     if(track2Playing == false)
     {
       track2Playing = true;
       audio.clip = track2;
       audio.Play();
     }
 
     fadeIn();
 }

}

function OnGUI() { GUI.Label(new Rect(10, 10, 200, 100), "Audio 1 : " + audio1Volume.ToString());

 GUI.Label(new Rect(10, 30, 200, 100), "Audio 2 : " + audio2Volume.ToString());

}

function fadeIn() {

 if (audio2Volume < 1) {

     audio2Volume += 0.1 * Time.deltaTime;

     audio.volume = audio2Volume;
 }

}

function fadeOut() {

 if(audio1Volume > 0.1)

 {
     audio1Volume -= 0.1 * Time.deltaTime;
     audio.volume = audio1Volume;
 }

}