Sorry TS, I haven't been purposely ignoring you ( I enjoy the exchanges) I've just been all over the place lately with work, here, and everywhere...wearing a lot of different hats right now LOL.
Anyway, I hope this statement will clear things up. Since you understand philosophy, when I talk about the nature of things, I'm basically referring to Kantian things-in -themselves.
I believe Kant was right, humans don't know thing-in-themselves. AKA the nature of existence. I think that's one reason why he made a big deal about critiquing pure reason. Pure reason of course, is a priori formal logic. It's central to the ontological argument for the existence of God as we know. It's meaningless. It leaves out experience (human sensory experience/ phenomena/cognition, etc.).
However, even though human phenomena won't tell us for sure about the nature of things, it gives us clues to the likelihood or plausibility for reasonable theories about same.
So, that leads to your question about justification for a claim of belief. Does my personal belief suffer from similar difficulties? Sure it does. But I'm not a Fundy either.
I won't digress too much about the value of the Religious Experience that has been well documented from the likes of Maslow, William James, Analytical Psychology/Carl Jung, NDE phenomena, et al. since that is a subject for another day. However, it is very impactful to say the least.
Music theory. Back In school, I had a debate with my professor about music theory. I didn't want to learn it because it was too hard. I was an ear trained musician. After much argumentation, we talked after class. He said, 'yes you're right Jim, I didn't want to tell you that music came before theory because if I told you that it would make you guys not want to learn theory.'
So yes firstly, I believe the phenomenon of music came first, then someone figured it out (theory). (There are minor exceptions in classical music... .) Secondly, to your point, there does not seem to be a clear answer, only a 'belief' as to the nature of it. In any case, what we do know is, that it doesn't confer any biological advantages. And we know the sounds of music itself takes primacy over music theory.
And as far as its second cousin, mathematics, for some reason my Kantian intuition tells me mathematics has an independent existence.