youtube-dl audio only code example

Example 1: youtube-dl mp3 only

youtube-dl -x --audio-format mp3 <youtube-link>

Example 2: youtube-dl download audio

youtube-dl -F http://www.youtube.com/watch?v={video ID}  to get available audio
then
youtube-dl -f {number of available audio} http://www.youtube.com/watch?v={video ID}