def _get_audio_bitrate(rep, index: int=None):
    """
    @TODO: add documentation
    """
    if rep.bitrate.audio_ is not None and rep.bitrate.audio_ != 0:
        opt = 'b:a' if index is None else 'b:a:' + str(index)
        return {opt: rep.bitrate.audio}
    return {}
