_pause(audio, fadeDuration) {
    if (audio instanceof audio_object) {
      return audio.pause(fadeDuration);
    }
    return Promise.resolve();
  }
Pauses an audio file and returns a promise that resolves when the audio is faded in/out.
NOTE: do not call this method directly, use pause_music or pause_effect instead.
@private
@param audio
@param [fadeDuration=0]
@returns {Promise}