Downloading youtube audio and converting mp3 is very simple now thanks to these guys: https://rg3.github.io/youtube-dl/

Installing:

Contents:

Usage: sh GetPlaylist.sh [youtube playlist URL] [output_directory]

Example:

To get the youtube playlist URL view the playlist by clicking on its name, no videos will be playing, then click on share and the url will be highlighted:

youtube-playlist
click on share and the URL will appear