code: function(event, fn, id) {
        var self = this;
  
        // Setup the event listener.
        self.on(event, fn, id, 1);
  
        return self;
      }
docs: Listen to a custom event and remove it once fired.
  @param  {String}   event Event name.
  @param  {Function} fn    Listener to call.
  @param  {Number}   id    (optional) Only listen to events for this sound.
  @return {Howl}