I get: "Youtube switched to separate audio/video streaming technology for higher quality formats and no longer provides direct 1080p/mp3 streams. Converting these streams to downloadable formats for you needs powerful server infrastructure, therefore 1080p/mp3 downloading has been limited to PRO users.

Read more at - https://videodroid.org/pro_upgrade.html" when trying to download mp3 audio track. You know what, developers?! Get a 1 star.