def fill_subparser(subparser):
    """Sets up a subparser to download audio of YouTube videos.

    Adds the compulsory `--youtube-id` flag.

    Parameters
    ----------
    subparser : :class:`argparse.ArgumentParser`
        Subparser handling the `youtube_audio` command.

    """
    subparser.add_argument(
        '--youtube-id', type=str, required=True,
        help=("The YouTube ID of the video from which to extract audio, "
              "usually an 11-character string.")
    )
    return download