Record stereo audio with AVAudioSession

// How to set up recording from the built-in mic

private func enableBuiltInMic() {
    ...   
    // Find the built-in microphone.
    guard let availableInputs = session.availableInputs,
          let builtInMic = availableInputs.first(where: { $0.portType == .builtInMic }) 
    else {
        print("The device must have a built-in microphone.")
        return
    }  
    ...   
    do {
        try session.setPreferredInput(builtInMic)
        ...  
    } catch {
        ...
    }
}

// Configure stereo recording

func selectDataSource(...) {
    ...
    // Set the preferred polar pattern to stereo.
    try newDataSource.setPreferredPolarPattern(.stereo)

    // Set the preferred data source and polar pattern.
    try preferredInput.setPreferredDataSource(newDataSource)
        
    // Update the input orientation to match the current user interface orientation.
    try session.setPreferredInputOrientation(orientation.inputOrientation)
    ...
}

// When to select a data source & updated the stereo input orientation

override func traitCollectionDidChange(_ previousTraitCollection: UITraitCollection?) {
    updateDataSource()
}

@IBAction func updateDataSourceSelection(_ sender: Any) {
    updateDataSource()
}

private func updateDataSource() {
    // Don't update the data source if the app is currently recording.
    guard controller.state != .recording else { return }

    let dataSourceName = dataSources[dataSourceChooser.selectedSegmentIndex]
    controller.selectDataSource( named: dataSourceName,    
        orientation:Orientation(windowOrientation)) { layout in
        self.layoutView.layout = layout
    }
}