[Tom Thistlethwaite]

I had this problem too – and it does appear to be a phase issue from a mono source split into an out of phase stereo pair when recorded. With headphones or speakers you don’t notice it, but when an iPhone recombines the two to play out of it’s mono speaker I think you get the garbling. By deleting either the right or left channel and then centre panning it, the problem should be solved.