I just switched from Android to iPhone XR and started a free 1 month trial of Apple Music. I only get 1 month because I have had apple music before but switched to google play music. I like Apple Music a lot, but there is one major limitation that might force me to have to cancel. I can't add music from my computer to my iPhone or iPad. I have a few concert DVD's that I extracted the audio from. I was able to add them to google play music but cannot do it with Apple.
You used to be able to do that. It's confusing. You likely still can do it. I have some tunes I want to put in there, but just haven't gotten around to it.
Can't you play those MP3's on your computer on iTunes and then plug in your iPhone or something and transfer them as "downloaded" music?
Surely Apple does not prevent you from listening to your own MP3's - I mean that's ludicrous. Of course you can do it - but maybe you just gotta think it through, and I honestly don't have the answer but would bet it exists and is pretty simple to do.