onended(callback) {
    this._onended = callback;
    return this;
  }
Schedule an event to be called when the soundfile
 reaches the end of a buffer. If the soundfile is
 playing through once, this will be called when it
 ends. If it is looping, it will be called when
 stop is called.

 @method  onended
 @for p5.SoundFile
 @param  {Function} callback function to call when the
                             soundfile has ended.