Un- / Mutes the AudioSource. Mute sets the volume=0, Un-Mute restore the original volume.

	// Mutes-Unmutes the sound of this transfor each time the user presses space.
	function Update() {
		 if(Input.GetKeyDown(KeyCode.Space)) {
		 	if(audio.mute)
				audio.mute = false;
			else
				audio.mute = true;
		}
	}

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour {
    void Update() {
        if (Input.GetKeyDown(KeyCode.Space))
            if (audio.mute)
                audio.mute = false;
            else
                audio.mute = true;
        
    }
}

import UnityEngine
import System.Collections

public class Example(MonoBehaviour):

	def Update() as void:
		if Input.GetKeyDown(KeyCode.Space):
			if audio.mute:
				audio.mute = false
			else:
				audio.mute = true