In Afrotistic, Noa hears Samuel humming "Roar." That provides some of the inspiration for the name of her organization, the Roaring Pebbles.
Joel Suzuki: In Fable of the Fatewave, Felicity has her wavemaker play the tune of "Firework" when she gets a call.
Diane from Love Over Gold was a fan of Katy Perry as a teenager, and has a poster of her in her childhood bedroom. She later realises that Katy Perry was one of her first crushes.