Is this a joke? You don't "buy" that she can't hear a difference? So what do you think she's doing? Faking?

I have a suggestion. Have a look into a foreign language other than English, which you've never encountered before. Say... Swedish. If you can tell the difference in regular, normal speed everyday speech between their "u", "y", and sometimes "i", even after a few months of "proper" tution, I'll buy you a "having a knack for languages" cookie.