This script uses FFMPEG for conversion which means that it needs to be installed on the system.
$ sudo apt-get install ffmpeg
And the mp3 codec is not included(I think)
$ sudo apt-get install libavcodec-extra-53
Or
$ sudo apt-get install libmp3lame0
Install from npm:
$ npm install -g ytmp3
To download and convert a youtube video to mp3
$ ytmp3 -u http://www.youtube.com/watch?v=sFrNsSnk8GM -f mp3
Where option -u is the url to the youtube video & -f is the format (mp3 or flv).