Use Browser Extensions to Play YouTube Audio Only
Extension for Chrome
Audio Only YouTube is a browser extension for Google Chrome which allows you to disable the video streams on YouTube and play YouTube audio only.
To enable or disable the functionality, you can simply click on the icon visible in your extension bar. This will disable the videos, allowing you to save over 50% of internet data as compared to when watching videos. You may find this useful not only for music, but also for different presentations, talk shows, podcasts, or discussions. This extension is an open-source project licensed under GPLv3. Please, note that it doesn't support Youtube live videos.