addSoundEffect(sfxIdentifier, looping, duration) {
    if (sfxIdentifier != null) {
      let sfxData = this._sfxDataByIdentifier[sfxIdentifier];
      if (sfxData == null) {
        sfxData = new SoundEffectData(sfxIdentifier, looping, duration);
        this._sfxDataByIdentifier[sfxIdentifier] = sfxData;
        this._sfxPlaylist.push(sfxData);
      }
    }
  }
Adds a sound effect to the sequence. A sound effect may be added to a playing sequence.
@param {String} sfxIdentifier path to sfx
@param {Boolean} [looping=false] whether to loop infinitely (only works on last sfx in sequence)
@param {Number} [duration] forced duration in seconds before moving to next in sequence