PBA: I believe R’ Moshe has a T’shuvah like that, where he says Kol Isha of a P’nuya is Muttar. Or maybe that’s a Hava Amina and he backs out of it out of respect for the Mishnah B’rurah? I don’t remember. (I think we assume the earliest age we need to worry about there being a Mi’ut Hamatzui of girls hitting puberty is 9 or 10.)
WIY: That’s ridiculous. People come up with similar musical compositions all the time.