AVAudioRecorder woes on MacOSX - objective-c

Question

I'm attempting to use AVAudioRecorder on MacOSX 10.14.x to get audio level readings of an input source -- in this case, the microphone. As this isn't on iOS, the use of AVAudioSession does not apply. I did enable the microphone in the SandBox settings, as well as double-checking everything in Audio Midi Setup app, with the microphone set to 2ch 24-bit Integer 44.1kHz. I've removed all error checking, but everything should be fine (eg. apparent success on all calls to the recorder object). Also, I tried this on another 10.14.x machine with the same result.
When I run this, all I get is values of -120.0 for both the average and peak power in my callback, even if there is audio.
Any idea what I'm doing wrong?
Note: When I initiate [recorder prepareToRecord], I receive these two error messages:
no saved enable hardware sample rate converter preference found
ca_debug_string: inPropertyData == NULL
AVAudioRecorder *recorder; // instance variable
NSTimer *levelTimer; // instance variable
NSURL *url = [NSURL fileURLWithPath: #"/dev/null"];
NSDictionary *settings = [NSDictionary dictionaryWithObjectsAndKeys:
[NSNumber numberWithFloat: 44100.00], AVSampleRateKey,
[NSNumber numberWithInt: kAudioFormatMPEG4AAC], AVFormatIDKey,
[NSNumber numberWithInt: 1], AVNumberOfChannelsKey,
[NSNumber numberWithInt: AVAudioQualityHigh], AVEncoderAudioQualityKey,
nil];
NSError *error = nil;
recorder = [[AVAudioRecorder alloc] initWithURL:url settings:settings error:&error];
[recorder prepareToRecord];
recorder.meteringEnabled = YES;
[recorder record];
levelTimer = [NSTimer scheduledTimerWithTimeInterval:0.03
target:self
selector:#selector(levelTimerCallback:)
userInfo:nil
repeats:YES];
- (void)levelTimerCallback:(NSTimer *)timer {
[recorder updateMeters];
float peakPower = [recorder peakPowerForChannel:0];
float avgPower = [recorder averagePowerForChannel:0];
}