As of iOS 10.0 all apps that access any of the device's microphones must statically declare their intent to do so. To do this, apps must now include the NSMicrophoneUsageDescription key in their Info.plist file and provide a purpose string for this key.
If an application attempts to access any of the device's microphones without a corresponding purpose string, the app will exit.
NSMicrophoneUsageDescription key - lets you describe the reason your app accesses any of the device's microphones. When the system prompts the user to allow access, this string is displayed as part of the alert.
<string>MyCoolApp uses the microphone to make you sound like Geddy circa '76.</string>