How can I extract audio from a YouTube video?

install Youtube-dl and FFmpeg then use the following commandline

Download the video

youtube-dl --extract-audio --audio-format mp3 http://www.youtube.com/watch?v=YOURVIDEO 

optionally use the -k flag to keep the video by default it deletes the video

I have had success with video2mp3.net – it is free and online!

Add the YouTube to MP3 firefox add-on.