[greengarstudios]

They do this already. It allows the phone to detect whether you're in the same place as someone else: if two phones are hearing similar sound, then you're likely to be next to each other. Color shows you photos taken by people near you. Audio is one signal for detecting who's near.

Note: GPS sometimes doesn't work indoors, where there's no line of sight to the satellites in the sky. Cell towers and Wi-Fi work, but aren't as precise.