AVCaptureDevice.DiscoverySession().devices not returning any devices

I have the following Swift code:

let devices = AVCaptureDevice.DiscoverySession(deviceTypes: [.builtInMicrophone, .externalUnknown], mediaType: .audio, position: .unspecified).devices
print("DEVICES: \(devices.count)")

However, when run it prints "DEVICES: 0" to the console. I'm running Zoom when I run this code, and I see the orange dot in the menu, so I know the mic is in use.

What am I missing?

EDIT: I'm building an app that plays a sound but, I don't want to play that sound if the mic is hot. For example; if the user is on a Zoom call then playing the sound would be distracting to the meeting. So I'm thinking if I can determine that some other app is using the mic (I don't care what for and don't need to access the mic itself) then I can temporarily disable playing the sound file.

First thing, you have to make sure you have (or the user accepted) the permissions to access the microphone. You can check and request microphone permissions using the AVAudioSession class, also try to verify the permissions you have granted in the General > Your application section.
Another thing you can check if the microphone is actually available for use by your application. Sometimes there are applications, like Zoom in your case, might be using the microphone exclusively, preventing your code from accessing it.

If you can expand your code, you can show how you are using the AVAudioSession class. You can use the requestRecordPermission() method to request permission.

Also, in order to verify if there is another application who is taking exclusive control over the microphone, you might use:

func isMicrophoneInUse(by application: String) -> Bool {
  let audioSession = AVAudioSession.sharedInstance()
  let category = AVAudioSession.Category.record
  let options = AVAudioSession.CategoryOptions.duckOthers

  do {
    try audioSession.setCategory(category, options: options)
    let activeMicrophone = audioSession.currentRoute.inputs.first
    return activeMicrophone?.owningApplication == application
  } catch {
    return false
  }
}

or in a more complicated way, you can use the AVAudioSession.routeChangeNotification notification to receive updates when the audio route changes, which may indicate that another application has taken control of the microphone.

Hope it helps