>>11112125I think the amount of non-trivial math - as a math student would understand it - in music theory is very limited. That is to say, the math<=>music connection is probably overstated.
That said, there's a lot of music theory that one will grasp quickly if one is acquainted with mathematics. This may also be the reason why music people who learn (about) math to understand music better might end up thinking that there's a lot of math in music (not knowing that there's just a gigantic bulk of mathematics far removed and more complicated than the one you need for music).
I personally only learned to play basic flute and harp, so my music background is extremely limited. If you want a few resources I found entertaining at least, I came across various youtube channels who at least point to some connections. E.g. if you consider
https://youtu.be/Ur6GOoSNGN0There is a small but interesting subgenre of youtube videos dealing with music theory that I want to recommend, even if it's not all that mathy. E.g. I really like this one
https://youtu.be/MdZSOoOF5MsComing back to proper math, of course there's all the mechanics and harmonic analysis that in principle relates to acoustics and thus math - but I should probably not count that.
I have not read it, and it might just be that it's the work of a mad man, but there is a 1000 pages treaty discussing music via topos theory
https://www.amazon.com/Topos-Music-Geometric-Concepts-Performance/dp/3764357312You can read the table of contents here
https://www.researchgate.net/publication/257259829_The_Topos_of_Music_Geometric_Logic_of_Concepts_Theory_and_PerformanceThat one might be hard to read just for not knowing topos theory.
Btw., for those interested in Topos Logic, this week I came across 2018 lecture notes that seem to be put together nicely, here
https://tcsc.lakecomoschool.org/files/2018/06/Como2018.pdf 
