because of recent changes to the Youtube API, the backend can't deliver the audio file anymore. So the example on ytaud.io does not work anymore.
An audio player for YouTube videos: https://ytaud.io/
YTAudio is a progressive web app that allows you to listen to YouTube videos in the background. It runs in the browser and uses modern browser-interfaces (like the MediaSession API, the Web Share API and the Share Target API) in combination with a ServiceWorker and a Web App Manifest to deliver an App-Like User Experience.