List all available formats of requested videos which -F

youtube-dl -F URL

Video format code, see the “FORMAT SELECTION” for all the info

youtube-dl -f 001 URL
youtube-dl -f "[height < 720]" URL

download a video as mp3 file

youtube-dl -x --audio-format mp3 URL

Where NUMBER is the starting and ending point of the playlist

youtube-dl -x --audio-format mp3 --playlist-start 1 --playlist-end 5 URL