This is done with around 50 lines of embedded JS code and some configuration data that's part of an audio pack, so people could come up with other implementations as long as there's enough context data coming from the base audio mod. For example, combat music:

#Atlyss #modding some experimenting with scripting for audio replacements; a bit buggy and full of edge cases, but it allows playing custom music in places where the vanilla game doesn't really support it, like subregions.

Ambulant needs a teammate IMO