You can register a sound event as usual, and then use AbstractTickableSoundInstance to play the sound once the boss spawns. That way you can stop the sound when the boss is defeated. Here is how Botania does it for their boss music 🙂
The website has a lot of useful information, but I don't quite understand why subscribing to this event can change an item's properties and how to do so. The concept 'Event' is quite hard to understand:(
I really don't know if I need the registry to hold anything. I just want it so I can put emblems under the tags folder. If I can't do that, I won't. My pre-made emblem patterns currently are not registered and are simply textures. I ran into problems with this method when creating a menu with all the patterns. I need some way to store them, and I want them to be easily expandable. I thought of config files, but datapacks seem more user-friendly. If the menu only appears in-game when you click on the special block, then I don't see a need to register anything. The packs are already loaded at that point, so my idea was for the menu to access the list of patterns, whether it be in a JSON or config file, to get whatever information it needed. I prefer to go the datapack route, so if I need a registry to create a new tag type, can I create one without filling it?