How to add new sound using this.game.add.existing(this)

Phaser 2 / CE
1

I create a class ------

class Bgm extends Phaser.Sound {
  constructor({ game, key, volume, loop }) {
    super(game, key, volume, loop);

    // this.game.add.existing(this);
  }
}

and I new it in a state with ------

this.game.bgm = new Bgm({ game: GAME, key: "bgm", volume: 0.5, loop: true });

instead of ------

this.game.bgm = this.game.add.audio("bgm", 0.5, true);

however, I cannot add it to game just like image or sprite using ------

this.game.add.existing(this); // in my class

how can I add audio using such ES6 style?