Problem With Walking Sounds

When i use this code footsteps gets bugy when you press W and D at the same time walking sounds not playing sometimes.

 #pragma strict
 var Walk : AudioClip;
  
 function Update()
 {
  
     if (Input.GetKeyUp (KeyCode.W))
 {
     audio.clip = Walk;
     audio.Stop();
  
 }
 
 if (Input.GetKeyDown (KeyCode.W))
 {
     audio.clip = Walk;
     audio.Play();
  
 }
 
 
     if (Input.GetKeyUp (KeyCode.D))
 {
     audio.clip = Walk;
     audio.Stop();
  
 }
 
 if (Input.GetKeyDown (KeyCode.D))
 {
     audio.clip = Walk;
     audio.Play();
  
 }
 
     if (Input.GetKeyUp (KeyCode.A))
 {
     audio.clip = Walk;
     audio.Stop();
  
 }
 
 if (Input.GetKeyDown (KeyCode.A))
 {
     audio.clip = Walk;
     audio.Play();
  
 }
     if (Input.GetKeyUp (KeyCode.S))
 {
     audio.clip = Walk;
     audio.Stop();
  
 }
 
 if (Input.GetKeyDown (KeyCode.S))
 {
     audio.clip = Walk;
     audio.Play();
  
 }
 
 
 
 
 
 
 }