My guess as to why the vanilla sins music keeps popping up is the fact that the reference file(hardcoded or not) is still looking up the default music for ambient & battle, I'm assuming that the music that waseem made has been put in the mod with their own custom names right? My suggestion is to instead name them what the default names for the vanilla sins music is, so that instead of it looking for the music where it usually is, it's looking for the "vanilla" music in the mod folder.
I have a list in the spoilers tag for all music file names(after all I have the music in my library):