RIAA, BPI, IFPI sue largest YouTube ripping site
torrentfreak.com
torrentfreak.com
Although google does not appear to be a plaintiff here, they have certainly indicated they could be one in the future ( https://torrentfreak.com/google-threatens-to-sue-huge-youtub... )
Taking audio from a youtube video is pretty simple code wise and it'd be a good idea to open source that maybe as a browser extension if someone hasn't already done it.