I doubt it’s a limitation of the microphone but rather that of the compression/digital to analogue conversion done to get data over the GSM network, no reason why a local app would be so limited. As I remember there was a really popular novelty cigarette lighter app around in the early days of the App Store that used inaudible acoustic signals to communicate between two iPhones that let you ‘pass on the flame’. Granted it was designed to work at close range.