Different audio on different surface script HELP

Hi! The thing I'm trying to do is to have different audio to play when you walk on different surfaces. I'm using a scipt I got on the forum but it's not working for me. I have different colliders in my scene with different tags depending on what surface it is but it not seems to work. Please check if something is wrong.

 //Written by Grady Featherstone
 //Create a variable to tell whether the player is walking or not
 var isPlayerWalking = false;
 var metalFloorSound : AudioClip;
 var grassDirtGroundSound : AudioClip;
 var riverStreamPuddlesSound : AudioClip;
 
 function Update(){
     //Check to see if the player is walking by checking for input from the walking keys
     if(Input.GetKeyDown("a") || Input.GetKeyDown("w") || Input.GetKeyDown("s") || Input.GetKeyDown("d")){
        //if those keys are pressed, the player must be walking...
        isPlayerWalking = true;
     }
     else{
        //if those keys aren't presesd, the player can't be walking.....
        isPlayerWalking = false;
     }
 }
 
 function OnCollisionEnter(collision : Collision){
     //Check if the player is walking...
     if(isPlayerWalking == true){
        //If the player is walking, check what sort of ground they're walking on, and then play that sound...
        if(collision.gameObject.tag == "metalFloor"){
          audio.PlayOneShot(metalFloorSound);
        }
        else if(collision.gameObject.tag == "grassDirtGround"){
          audio.PlayOneShot(grassDirtGroundSound);
        }
        else if(collision.gameObject.tag == "riverStreamPuddles"){
          audio.PlayOneShot(riverStreamPuddlesSound);
        }
     }
 }