Toggle Button for Sound On/Off

You can make your code independent of previous button state

Example :

Button Script :

 public GameObject audioSource;
 bool soundToggle = true;
 
 OnGUI()
 {
  soundToggle = !soundToggle;
  if(soundToggle)
  {
   audioSource.SetActive(true);
   //audioSource.mute = true;
   //audioSource.volume = 1.0f;
  }
  else
  {
   audioSource.SetActive(false);
   //audioSource.mute = false;
   //audioSource.volume = 0.0f;
  }
 } 

When the button is pushed toggle a boolean, set the AudioListener in your scene to inactive when the boolean is false.

http://forum.unity3d.com/threads/74698-Toggle-Sound-On-Off