Giggles.
It's a sound he loves the most. A sound that's unmistakable to him. He knows every pitch and he knows the meaning behind it.
He loves being the reason for that sound. He loves knowing that what people hear is because of him. He knows countless different ways to release the sound and he knows they're ways nobody else knows.
He knows that he's not the only one who likes that sound. People all over the world love it too, but they never sound as good as what John Paul hears. He loves that. He loves hearing Craig's giggles.