Clark's hearing supposedly got better because he lost his eyesight, right? I think that was the theory. But when you lose your eyesight, aren't all of your other senses increased? Smell, taste, feel, in addition to hearing?
It was the interaction between the kryptonite and Clark's heat vision that ruined Clark's retinas. But at the same time that the kryptonite passed through Clark's vision, the kryptomutant was also doing the high-pitched sound. A, the kid was able to do the high-pitched sound because of an earlier interaction with kryponite, and honing the skill it gave him. B, that brooch with the kryptonite was involved. What if it wasn't just the loss of his vision, but also the interaction of the kryptonite that caused Clark's superhearing to develop? If it were just the loss of his vision, then Clark should've also developed supersmell, supertouch, and supertaste, if he has those. I don't know comic canon but I know that in Lois & Clark, Clark had supersmelling abilities. So I don't think it could've just been his loss of sight.