static filterVariantsByAudioLabel_(variants, preferredLabel) {
    return variants.filter((variant) => {
      if (!variant.audio || !variant.audio.label) {
        return false;
      }

      const label1 = variant.audio.label.toLowerCase();
      const label2 = preferredLabel.toLowerCase();
      return label1 == label2;
    });
  }
Filter Variants by audio label.

@param {!Array<shaka.extern.Variant>} variants
@param {string} preferredLabel
@return {!Array<shaka.extern.Variant>}
@private