The automatic expansion files are a great job and made it possible to release Telling Eksjo easily. Unfortunately on Iphone you’re prevented to download > 100MB over 3G/4G by App Store, and the other reason is there will be more guided tours so that the number of files will be even bigger. Audio size/quality is already optimal and can’t be reduced, and separate apps per language is not an option for this client.
I can certainly download files from a first-time splash screen, but there are considerations. It must be fail-safe and resume download if the internet connection is broken, handle not enough space on the phone, etc.
I have to use mp3s, since it’s for Iphone and Android. Should I just download the .mp3s, can I play them from a .zip, will an app that downloads mp3s be forbidden by the stores, will it help to rename the extensions, etc.
I was planning to create a folder tree in System.DocumentsDirectory to download them into. Should be no problem?
Note that the app must download all files for a language before it can be used, because it should still be possible to download the big files over free WiFi and then not use internet at all anymore.
Any good advice you may have to share from experience with this 
Edit: first of all, can I play sounds that are not in /assets at all? (On both Iphone and Android)
Edit 2: Ideally, to simplify error handling, I would download one big zip file with a folder structure and files and “extract to documentsDirectory”. Has someone done this and do you recommend it?