On Collision, Play Sound If Button Is Pressed

Hi

I was looking to have the game play a walking sound while only on a certain object(s), in this case, a wooden pillar. I wanted to write a script where when the player was on this wooden pillar, the sound would play if he moved around, in this case pressing W.

Heres the script I have so far, feel free to let me know if i was way off or not

If you need any additional info, let me know!

var StepSound : AudioClip;

function OnCollisonEnter(collision : Collision)
{
	if (other.tag == "pillar")
		{
			function Update() 
			{
			
				if (Input.GetKeyDown (KeyCode.W)) 
					{ 
						audio.clip = StepSound;
						audio.Play();
					} 
				if (Input.GetKeyUp (KeyCode.W))
					{
						audio.Stop();
					}
			}
		}
}

See if this helps
http://www.lynda.com/home/Player.aspx?lpk4=109295&playChapter=False
@ time 4:40