If you want to get sound from a youtube video and save it in mp3 format go to http://vixy.net/ , paste the youtube video url, choose the format you want to convert the video to (in this case mp3), and hit the Start button, wait for the conversion to finish, then download the mp3 file.

That’s it, you are done!

Another site you can use for exactly the same purpose is http://www.convertmytube.com/