I have some like this http://rec.mro.name/podcasts/zuendfunk/broadcasts.atom (and .rss, too) which plays the audio nicely e.g. in https://miniflux.app but I remember having trouble with iTunes on MacOS and the Apple podcast iOS app.

But his is mostly try-and-error and merely no specs. Apple doesn't talk about atom AFAIK.

Racket is interesting, I'm into Ocaml, but create those feeds via https://codeberg.org/mro/internet-radio-recorder/src/branch/...