audio_engine = function () {
  this._pool = {};
  this._sfx = [];
  this._music_stopping = [];
  this._voice_stopping = [];
  this._update_waiting_effect_for_interaction_bound = this._update_waiting_effect_for_interaction.bind(this);

  // ensure volumes are updated (Needed for cocos in case no profile will sync yet to trigger updating volumes)
  this._update_volume();
}
audio_engine - Engine for playing music and sfx.