I tried two different methods:
First, I used the website convertio.co. This worked fine for MP3 to OGG, and the resulting M4A files worked fine on my desktop. I considered that might be the issue, so I also tried converting via Audacity - still no dice.
Additionally, the stock sound effects...
SO audio in my game works fine on desktop but won't show up at all on my iPhone, in either Safari or in Chrome.
I've provided all necessary M4A files and made sure there aren't any spaces in filenames.
I'm using all default sound effects, so you'd think those would show up at least?