Yanfly's Musicmenu plugin not working.

// Addition to Music Menu plugin to add to the main menu
(function() {

  // You can change this. Keep it inside quotes!
  var COMMAND_NAME = "Music Menu";
  // You can change this. It's the order it will appear in the menu.
  // The number 4 means it will be the 4th item in the list
  var ORDER = 4;
 
  // Don't change anything below this line.
 
  if (!Window_Command.prototype.addCommandAt) {
  Window_Command.prototype.addCommandAt = function(index, name, symbol, en, ext) {
      if (en === undefined) enabled = true;
      if (ext === undefined) ext = null;
      var obj = { name: name, symbol: symbol, enabled: en, ext: ext};
      this._list.splice(index, 0, obj);
  };
  }
 
  Window_MenuCommand.prototype.addMusicCommand = function() {
      this.addCommandAt(ORDER+1, COMMAND_NAME, 'music', true);
  };
 
  var _makeCommandList = Window_MenuCommand.prototype.makeCommandList;
  Window_MenuCommand.prototype.makeCommandList = function() {
     _makeCommandList.call(this);
     this.addMusicCommand();
  };

  var _createCommandWindow = Scene_Menu.prototype.createCommandWindow;
  Scene_Menu.prototype.createCommandWindow = function() {
    _createCommandWindow.call(this);
    this._commandWindow.setHandler('music', this.commandMusicMenu.bind(this));
  };
 
})();