Reason he can't hear, probably:
1. Twilight style: ShE'S sPeCiAL
2. She just doesn't talk in her brain (because she's mute), probably thinking in 'feels'? pr anything other than 'words'.
She's SPECIAL as in she also can "read" minds, as in people's true thoughts/words transformed into written words to her eyes. Too bad she's kinda illiterate/words too blurry that she can't understand them.
This story reminds me of that theory about people either hearing/listening to voices or form/seeing word sentences in their mind when they think internally/talking to themselves.