Walking sound change

Hello!

I have this script for the walking sound, but it doesn't work. What's wrong?

 #pragma strict
      
 var WalkingSound : AudioClip;
      
 function Update (){
 if(Input.GetButton("Horizontal") || Input.GetButton("Vertical"))
 {
     PlayerWalkSound();
 }
 }
      
 function PlayerWalkSound(){
 yield WaitForSeconds(1);    
 audio.volume = 0.5;
 audio.clip = WalkingSound;
 audio.pitch = Random.Range(0.8f, 1.0f);
 audio.Play();
 }

I must do it with yield instead of

 if(!audio.isNotPlaying)
 {
 //stuff
 }

because the time between the footstep sounds must be changeable cause of the different speeds. (sprint, crawl...)