def get_audio_itag(stream_lst,
                   abr='128kbps',
                   subtype='mp4'):
    """
    Return the `itag` of a YouTube video with specified audio bitrate (`abr`) and subtype.
    If the desired `arb` does not exist, the user is prompt to input a new one from a list.

    Input:
      stream_lst:  list of available media formats
      abr:         desired bit rate, string of the form `xxxkpbs`, where `x` is a number
      subtype:     desired subtype, string -- available options are `mp4` (default) and `webm`
    Output:
      `itag` of YouTube object
    """
    audio_streams = [stream for stream in stream_lst if stream.includes_audio_track == True
                     and stream.includes_video_track == False]
    audio_abrs = [stream.abr for stream in audio_streams if stream.subtype == subtype]
    if abr not in audio_abrs:
        print('Select a new abr variable from the following list: ', audio_abrs)
        new_abr = input()
        return get_audio_itag(stream_lst, new_abr, subtype)
    itag = [stream.itag for stream in audio_streams if stream.abr == abr]
    audio_itag = itag[0]
    return audio_itag