This service is neat: http://huffduff-video.snarfed.org/

It extracts the audio then "huff-duffs" it; i.e., puts it into an RSS feed you can consume with your podcatcher of choice. Check it out, it's great.

My side project TubeSock (http://stinkbot.com/index.html) can do this for you. Just open the preferences and set it to download as MP3 or AAC.

There are lots of websites that will extract mp3s from youtube videos which you'll find if you just google "youtube to mp3". There are also browser extensions which do this. Here's one:

http://www.video2mp3.net/extension.php

If its android you can try this http://www.snaptube.in/

I use it for everything including X rated content from adult sites

youtube-dl can do that.