How to convert YouTube videos to mp3 on iPhone?

I have a iPhone 5s running iOS 7 and would like to store mp3s of YouTube videos. First, I need to know of a method to convert these files. Secondly, I would like to know of how I can easily view the URL of these videos. I have difficulty doing so when I'm redirected to YouTube's mobile website. My internet access is limited to my phone only.

YouTube-mp3.org works we'll use it in your mp3 app