Loading Audio

You can use !this.load.isLoading() instead

playing = false

create() {
  this.load.audio('song', ['assets/sounds/0781.ogg'])
  this.load.start()
}

update() {
  if (!this.playing && !this.load.isLoading()) {
    this.playing = true
    this.sound.play('song') // this works
  }