We still don't know how the change happens, maybe you just wake up one morning and you are a catulus? Ears gone and all. Or it varies, in chapter one Riko said that Yuki was the type of beastkin without human ears. Which implies the existence of beastkin (vague term btw) with human ears.
The human disguise pills Maron was using probably replaces the cat ears with human ones, for instance, how that works with her sense of hearing is another question.
Or it might just be that trying to make sense of things like this is a waste of time and the answer was handwaving by a higher power (the author) all along. xD