I had a downloader that worked pretty well, but it stopped working a few days ago due to a YT update, so I gave this one a try. It does everything I need it to do, and nevermind the fact that it asks you to buy the upgrade to DL the Mp3 files. An online search for "audio file extractor" will steer you toward an add-on that can pull an Mp3 out of the Mp4 videos this add-on lets you download. It's an extra step I know, but if you, like me, are flat-broke, you learn to make-do.

For what it's worth, the one-click green button right there on the YT page is nice and convenient.