How to stop Audio

 function Update () 
 {
     if (Input.GetKeyDown ("w"))
     {
         audio.Play ();
     }
     else (Input.GetKeyUp ("w"))
     {
         audio.Stop ();
     }
 }

I know its messed up at the Stop. I need to know how to make it to where when I release "w" the audio stops. Right now when I press "w" and release it the sound continues.