I recently watched Wifi Ralph. There is a scene where
Disney princesses teach Vanellope a mechanism that allows her to unlock the wonderful, magical power of diegetic music.
Didn't want to read the spoiler? No problem, here goes an explanation. Diegetic music is a part of the soundtrack for audiovisual work that is generated within the world and thus is heard by the characters.
Usually, in Disney works, the power of diegetic singing seems to be ubiquitously present in populations (think of Belle and Moana's introduction songs, in which whole villages harmonize into it). Yet not all characters are able to sing properly (i.e.: Donald Duck, whose bad, tone deaf singing can even be weaponized), suggesting that this ability is a genetic trait rather than a physics paradigm of such worlds.
Characters that can break into song at will and in any place usually have orchestral backing to their singing. This means they have an internal, organic jukebox which can mimic most (if not all) musical instruments, and which is loud enough to be perceived as such. How would this fit into their anatomy? What would the caloric costs involved be?
For the purpose of this question, I am ignoring the fantastical powers of bending light and controlling the elements which usually accompany diegetic songs (no walking through the air because you know the colors of the wind, for example).
For the purpose of this question, I am also assuming that all characters capable of diegetic singing are gifted with absolute pitch.
Part of the Anatomically Correct Series.