Sound not working?

I am VERY stuck on this , so i want to play a foot sound when i go over the collider , but this comes up : No appropriate version of 'UnityEngine.AudioSource.Play' for the argument list '(UnityEngine.AudioClip)' was found.

The script

 #pragma strict
 
 private var footsteps : PlayerFootsteps;
 private var footSound : AudioClip;
 
 function Start()
 {
     footsteps = GameObject.Find("First Person Controller").GetComponent(PlayerFootsteps);
 }
 
 function OnTriggerEnter (Col : Collider)
 {
     if(Col.gameObject.tag == "Player")
     {
         footsteps.inWater = true;
         audio.Play(footSound);
     }
 }
 
 function OnTriggerExit (Col : Collider)
 {
     if(Col.gameObject.tag == "Player")
     {
         footsteps.inWater = false;
         audio.Stop(footSound);
     }
 }