It doesn’t take long for Despereaux’s siblings to give up on educating him, which gives Despereaux the freedom to do what he wants. He stares at the light coming through windows and reads the story in the library many times. He also finally discovers what the “honey-sweet sound” is: music, and specifically, King Phillip playing his guitar and singing to the Princess Pea every night. Despereaux hides in a hole in the Pea’s bedroom, listening to the music. It makes his soul “grow large and light inside of him,” and it sounds like heaven.
As far as Despereaux is concerned, his siblings giving up on him is a gift: now, he can dedicate his time to admiring beauty in the castle, wherever he finds it. And when Despereaux discovers that it's the human King Phillip making the music, he becomes increasingly sympathetic to people, something Furlough warned him not to do. The novel associates music with light and goodness as it describes Despereaux’s soul growing.
Despereaux sticks an ear out of the hole to hear better, and soon he’s all the way out of the hole. Despereaux might not follow most mouse rules, but he does follow the most important one: to never show oneself in front of a human. The music, however, causes Despereaux to ignore his mouse instincts, and soon the Princess Pea notices Despereaux and points him out to King Phillip. Phillip, however, is very nearsighted, so he insists the creature is a bug. As they argue, Despereaux trembles with fear. The Pea asks her father to keep playing to make Despereaux feel better, but Phillip suggests it’d upset the world order if a king played music for a bug. Finally, he agrees to play. Despereaux forgets his fear and creeps all the way to the king’s feet.
The novel continues to associate music with humanity. And as Despereaux overrides his mouse instincts to pursue the music, he, too, becomes more human. The Princess Pea seems to point Despereaux out in a purely neutral, if not positive, manner—that is, she’s not concerned at all about a mouse listening. This suggests that she’s open to new experiences and interested in being kind to others. King Phillip, on the other hand, shows that he’s closed off and unwilling to step outside what he believes is good and correct (like that kings shouldn’t play for bugs, or likely mice either).