http://www.techradar.com/news/best-youtube-to-mp3-converter

YouTube is home to thousands of videos (such as vlogs and music videos) that would make perfect audio files for offline listening. You could use a standard downloader to save the videos and then use a separate tool to extract the audio, but why go to such lengths when a free YouTube to MP3 converter can perform both tasks with a single click?

There are lots of websites that can strip the audio from YouTube videos and send you the audio file to download, but here we've chosen to focus mainly on desktop software. These programs have several advantages: they're faster than browser-based tools, they don't require you to submit your email address (and therefore leave yourself open to spam), and they often allows you to rip the audio from multiple videos at once.