How to make sound play once until called again

So, If my player jumps and lands, I want to make a thump noise like youn would if you landed on the ground. The problem I'm having is that I can't get it to only play once, and that is when you hit the ground. This is the script so far.

 #pragma strict
 
 var isGrounded : boolean = true;
 var sounds: AudioClip[];
 
 function Update () {
 
     var controller : CharacterController = GetComponent(CharacterController);
 
     if (controller.isGrounded)
 
     {
 
         isGrounded = true;
         if (audio.isPlaying) return;
         audio.clip = sounds[Random.Range(0,sounds.length)];
         audio.Play();
 
     }
     else
     {
         isGrounded = false;
     }
 
 }

I know the problem with this but I just don't know how to fix it. I can't just time the sound because if a player jumps of a hill I want it to make the sound when he lands not in mid air.