mono to stereo Audition

You do realise that this won't be 'proper' stereo, don't you? What you get if you convert a mono recording to stereo is what the BBC used to refer to as 'glorious'  mono - although why they picked that word, we may never know, as it was a long time ago now. But essentially it's the same audio on two channels, so if you play it on a stereo system it will come out centrally in the sound field, and any background atmospherics will also be central. That's fine - except that it takes up twice as much room in file size, and has no benefits whatsoever - if you create, say, a stereo interview from it in Multitrack, you can pan the mono tracks to wherever you want in the stereo field anyway. If they are dual-mono to start with, you can still pan them to get the same effect, obviously - but you've still doubled the amount of data storage requirements.

Anyway, converting mono to dual-mono (glorious mono) is very easy in Audition. Open your mono file, go to Edit>Convert Sample Type and set it something like this:

Click on OK and save the result. That will immediately double your file size.

As to why this is happening - well it's hard to say. It rather depends upon which iPhone you've got as well. The newer ones record in mono if you use the internal mic, but stereo if you connect an external one. So to get any further with that, we'd need a model number, I think.