Customize the volume increasing curve for subtle sound tweaks and maximum volume boosts.
< Feedback on YouTube Volume Curve Designer
Something else broke YT music in the last few days. Changing paper-icon-button.volume → tp-yt-paper-icon-button.volume in line 73 fixed it I think.
Small bug on YouTube Music: The script only gets activated when you start playing a song in the "video view" (where you have the video / album cover on the left and "up next" on the right) for the first time, but not when playing a song from the home page for example. After doing that once, it works everywhere.