I would suggest using booleans to control your music playing:

 // at the top with loudbang
 var loudBang : AudioClip;
 var touchedWall : boolean = false;
 
 // then in your OnTriggerExit() function
 function OnTriggerExit(col : Collider)
 {
     Debug.Log("The trigger fired");
     if (!touchedWall) // if touchedWall is false
     {
         touchedWall = true;
         audio.PlayOneShot(loudBang);
     }
 }
 
 // maybe in your OnTriggerEnter() function you should add
 function OnTriggerEnter(col : Collider)
 { 
     touchedwall = false;
 }