code: function(event, fn, id, once) {
        var self = this;
        var events = self['_on' + event];
  
        if (typeof fn === 'function') {
          events.push(once ? {id: id, fn: fn, once: once} : {id: id, fn: fn});
        }
  
        return self;
      }
docs: Listen to a custom event.
  @param  {String}   event Event name.
  @param  {Function} fn    Listener to call.
  @param  {Number}   id    (optional) Only listen to events for this sound.
  @param  {Number}   once  (INTERNAL) Marks event to fire only once.
  @return {Howl}