Frankie Messages: 10 Registered: September 2017 Location: DC
Chancellor
I don't know about an app, but Any Video Converter has always done the trick for me. Just a head's up, they will try to get you to download a security program, Bytefence or something like that - don't do it! Other than that, I've never had issues with this site before. I'm sure there is an app out there that does it too. I just haven't heard of one.
As far as the legality of extracting audio files from Youtube, it is legal as long as the video is public domain or you have the copyright owner's permission.