The series has used diegetic songs to elevate its story for decades
How Nobuo Uematsu composed FF7's "One Winged Angel"
Final Fantasy 7 Rebirth may have been slammed for playing too much with its multiverse and blowing up its runtime, sure, but it nailed a major part of Final Fantasy tradition: the diegetic soundtrack.
Diegetic songs, that is, songs that other characters can hear in a fictional universe, have been a staple of musical theater, film, and animation almost since those industries began—think of Snow White instructing the clueless animals to "some while [they] work ,” or Maria teaching the Von Trapp children “Do Re Mi” in The Sound of Music.
Final Fantasy first dabbled with them in Final Fantasy 6, when former Empire knight Celes sings the "Aria di Mezzo Carattere". Both players and in-game characters hear the song, and it's all part of a ruse to get Setzer and his airship to join the party. If the player cannot get Celes to sing the correct lyrics, they fail their mission and must start over. In the original 1994 version of the game, composer Nobuo Uematsu simulated a vocal track, but it resulted in an unintelligible synthesized "voice" harmonizing with the instrumental melody due to the technological limitations of the time. In the latest remaster, he managed to get a proper vocal track with lyrics.
