Finding out the ‘gist’ of the auditory system is in the upper brainstem (why babies born “without brains” can hear and react), I’ll travel down the EvoDevo here, but only after I make one last stop at mammalian hearing through “Visually Memorable Neuroanatomy for Beginners”, which I’ll peek at first.
https://www.sciencedirect.com/science/article/pii/S0960982216310661