Youtube-dl
Download a video :
youtube-dl -f best https://www.youtube.com/watch?v=14xMo-6Peho
List of all available formats :
youtube-dl -F https://www.youtube.com/watch?v=14xMo-6Peho
Docs :
https://github.com/rg3/youtube-dl/blob/master/README.md
FFmpeg
Convert a video in only audio file :
ffmpeg -i current-track.mp4 -ab 128k -ar 44100 current-track.mp3
Conversion example :
script example :
http://oriolrius.cat/blog/2013/09/20/converting-webm-files-to-mp3/
ginojava 