If ScreenSleeves Standalone doesn't appear to realise that there's music playing, then the answer is almost certainly linked to permissions.
With each release, MacOS has become more secure. As a general rule it won't do anything unless you've given permission.
Since Catalina, ScreenSleeves won't be able to find out what music is playing without you allowing it to access 'System Events' as well as whatever app(s) you use to play music. When you first run SS, you should see a dialog, and it's important to agree to these things.
Regardless of whether you remember the dialog(s) and what your answer was, you can always grant the necessary permission with a checkbox or two.
You need to go to System Preferences > Security and Privacy > Privacy > Automation