I was watching a concert and I wanted to take just the audio of a song, no video. I knew you could download the full audio from videos pretty easily with youtube-dl but now just wanted an specific portion. Thanks to these links (link1 and link2) I managed to get what I wanted:
$ youtube-dl --youtube-skip-dash-manifest -g "VIDEO_URL"
# copy the second url (audio) from the above command output
$ audio_url="AUDIO_URL_FROM_ABOVE"
$ ffmpeg -i "$audio_url" -ss 00:00:30 -t 00:05:20.0 -q:a 0 -map a sample.mp3
# PLAY IT!
$ vlc sample.mp3