toggle between mute / unmute
function() {
      if(!volume) {
        return;
      } else if (volume.muted) {
        p.unmute(function(err, status) {
          if (err) return;
          volume = status;
        });
      } else {
        p.mute(function(err, status) {
          if (err) return;
          volume = status;
        });
      }
    }