Reproduction steps:
1. Open the attached project.
2. Open the 'Scene0'.
3. Attach 'MainScript' script to the script slot on the 'Main Camera' game object in the Inspector.
4. Build and run it on the iOS device.
5. Click 'Ask for permissions'.
6. Select 'Don't allow'.
7. Minimize the application.
8. Go to device 'Settings > iosMicrophonePermissions (Scroll down)'.
9. Toggle 'Microphone'.
- Application will crash

frame #0: 0x0000000185f93224 libsystem_kernel.dylib`mach_msg_trap + 8
frame #1: 0x0000000185f9309c libsystem_kernel.dylib`mach_msg + 72
frame #2: 0x0000000186f64e90 CoreFoundation`__CFRunLoopServiceMachPort + 192
frame #3: 0x0000000186f62ae4 CoreFoundation`__CFRunLoopRun + 1060
frame #4: 0x0000000186e92da4 CoreFoundation`CFRunLoopRunSpecific + 424
frame #5: 0x00000001888fc074 GraphicsServices`GSEventRunModal + 100
frame #6: 0x000000018d146058 UIKit`UIApplicationMain + 208

• frame #7: 0x0000000100077474 microphonetest`main(argc=1, argv=0x000000016fd8f998) at main.mm:33

frame #8: 0x0000000185ea159c libdyld.dylib`start + 4

Reproduced with:
2017.1.3p2, 2017.2.2p1, 2017.3.1p4, 2018.1.0b11. 2018.2.0a4
iPhone 7 iOS 11.0.2, iPadMini 3 iOS 10.3.2, iPad Pro 10.5" iOS 10.3.3

Not reproducible with:
Android

Resolution:
Apple design choice. If privacy settings are changed app should be restarted (to make sure new settings take effect).