Download the audio from Youtube videos as mp3 file. This module uses youtube-mp3.org for the conversion.
var mp3 = require('youtube-mp3');
mp3.download('https://www.youtube.com/watch?v=bRmSMdQnfAc', 'LXJS 2013 Keynote', function(err) {
if(err) return console.log(err);
console.log('Download completed!');
});