I have made a browser extension that plays only the audio, blocking the video on YouTube, YouTube Music and all embedded YouTube videos in third party sites. It can also hide the thumbnails and avatars and skip the ads. It is compatible with Chrome, most of the Chromium browsers and Firefox. Additionally, it can be installed in smartphones using the Kiwi browser that supports extensions from the Chrome Web Store. I released the extension back in October, 2019 but from now the source code is available on GitHub as well.
