youtube-dl download 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}