Esmeralda doesn't know how much Quasimodo can hear, but he watches her when she sings.
When the songs from mass carry to the (silent) bell tower and she absently sings along, he looks at her as if hearing the hymns for the first time. When she stares down from the top of the world and feels particularly melancholy, and a sad tune passes her lips, Quasimodo's gaze always falls on her; soft and fascinated.
He might love the bells of Notre Dame more than anything in the world, but they have taken most of his hearing. And so Esmeralda doesn't understand how he knows whenever she's singing, or how well he can hear her when she does.
One evening, as she hums to herself and notices him watching, she asks.
"Can you hear it?"
Her signs are clumsy and slow, but she's learning.
Quasimodo makes a sound that's somewhere between thoughtful and annoyed (that's a bit of a complicated question) before signing back.
I can feel it.
That gives her an idea.
Esmeralda holds out her hand, and Quasimodo gives her a confused smile before allowing her to take his. He trusts her— more than anything— but he doesn't always understand her. She guides his hand to her neck and gently presses his fingers to her throat. His eyes widen in horror (his hideous hands don't belong on her delicate throat— what if he hurts her?) and he starts to pull away, but her voice stops him.
"It's alright."
Their eyes lock, and the fear in Quasimodo's is instantly replaced with surprise.
He can feel her voice.
Esmeralda can't help but smile as his jaw slacks, and she gives his wrist a gentle squeeze as she begins to hum. Now he's smiling too, eyes filled with emotion, and they drift shut as she begins to sing.
"Someday…"