Once a user has denied microphone access for your app, you cannot present them with the permissions dialog again. The saved settings are used. Instead you can prompt the user to go into their settings and make the change.
[[AVAudioSession sharedInstance] requestRecordPermission:^(BOOL granted) {
if (granted) {
NSLog(@"granted");
} else {
NSLog(@"denied");
UIAlertView *alert = [[UIAlertView alloc] initWithTitle:@"Microphone Access Denied"
message:@"You must allow microphone access in Settings > Privacy > Microphone"
delegate:nil
cancelButtonTitle:@"OK"
otherButtonTitles:nil];
[alert show];
}
}];