setupPreferredAudioOnSrc_() {
    const preferredAudioLanguage = this.config_.preferredAudioLanguage;

    // If the user has not selected a preference, the browser preference is
    // left.
    if (preferredAudioLanguage == '') {
      return;
    }

    const preferredVariantRole = this.config_.preferredVariantRole;
    this.selectAudioLanguage(preferredAudioLanguage, preferredVariantRole);
  }
This method setup the preferred audio using src=..

@private