Audio on mobile

I see lots of half finished threads on this topic, that always end with "This discussion is closed" right before we get the punch line. Anyway, so here's another one:

My course will contain a lot of audio and will be intended to be used on mobile devices almost exclusively. (These will probably be phones most often, not iPads.) I've tested the app on my PC and on my iPad, the audio comes through fine, but on the iPhone, there is no audio at all. All audio files are mp3. If I need to use wavs I can do that, but I'm just worried about cellular download speeds because the course will be used outdoors.

How can I get audio on my iPhone?