saving an audio file to my iphone

My wife has some mp3s that she wants to save directly to her phone.

We share a family iCloud account. She has some semi-large mp3s that she accesses fairly regularly, but even though they're accessible in the Files app, they still have to be re-downloaded every time she wants to play them. This has been using up her cel data.

Frankly, it's frustrating and kind of weird that I even have to ask this question: what's the best/easiest way to save a file to her phone that won't be "removed", so that she can quickly and easily access it without using up her data?

I was thinking maybe the voice or music memo apps, the Apple Music app or Podcasts app might be a solution here.

Any interesting suggestions? Ideally it would be dead simple to find and play the file.