While hearing ultrasonics may not be possible, apparently you can feel the effects of mixing ultrasonics. I found this article an interesting read. https://www.sciencenewsforstudents.org/article/feeling-objects-arent-there
Absolutely. Our brains are constantly struggling to get as much information as possible out of our sensory inputs. I'd bet if you looked back to the evolution of our hearing system you would find an advantage in being able to detect sub-sonic variations so we are still able to hear beats (even at very low levels).so seems to be in the brain.