Попробуйте это дополнительно, незадолго до [audioIO start]:
AVAudioSession *instance = [AVAudioSession sharedInstance];
NSLog(@"output channels: %ld", (long)instance.outputNumberOfChannels);
NSLog(@"max channels: %ld", (long)instance.maximumOutputNumberOfChannels);
BOOL r = [instance setPreferredOutputNumberOfChannels:instance.maximumOutputNumberOfChannels error:&error];
NSLog(@"set preferred channels: %d error: %@", r, error);
NSLog(@"output channels: %ld", (long)instance.outputNumberOfChannels);