SoundCoordinator.js

  /**
  * @param {GainNode} gainAmount
  */
  _turnMusicUp(gainAmount)
  {
    var music       = this.sound.getCurrentMusic();
    var currentGain = this.sound.getGainValue(music);

    this.sound.fadeIn(music, 1000, currentGain, gainAmount);
  }

  _duckAndLowerMusic()
  {
    var music = this.sound.getCurrentMusic();

    this.sound
      .fadeOut(music, 500, 1, 0.5)
      .callback(() =>
        this.sound.fadeIn(music, 1000, 0.5, 0.75)
      );
  }