makeManifestAudioOnly_() {
    for (const variant of this.manifest_.variants) {
      if (variant.video) {
        variant.video.closeSegmentIndex();
        variant.video = null;
      }
      if (variant.audio && variant.audio.bandwidth) {
        variant.bandwidth = variant.audio.bandwidth;
      } else {
        variant.bandwidth = 0;
      }
    }
    this.manifest_.variants = this.manifest_.variants.filter((v) => {
      return v.audio;
    });
  }
Modifies the current manifest so that it is audio-only.
@private