Momiji is in the habit of talking to himself, alone in his big, empty house.
Hatori checks in on him often (guilt, yes, but he appreciates it nonetheless), and sometimes his father even stops by (which he cannot always manage to appreciate, though he tries), but still it is too quiet.
So he fills it with talk and with music. He repeats his lessons aloud, announces what he will play, pretends he is thanking his audience after a concert.
Now that he knows to look for Momo hiding in the garden, he speaks even louder.
It's almost a real conversation.