Since I only found posts about this topic from a year ago or older, I wanted to check if there has been any progress for volume normalization when using a third party app (in my case Spotify).
The setting for it in the Spotify app is only applied on the device youre changing it on. Because those settings are not accessible in the Sonos app, there is no way to set it for my brand new Sonos setup (neither through Spotify nor Sonos).
The result is that the volume of the music varies frequently; often just slightly, but regularly its very noticeable. It happens often enough to degrade the user experience considerably.