AVAudioSession ducking stops when … | Apple Developer Forums

When I call

startWriting

for a video capture session, the ducking for my audio session stops, and the external audio goes back to full volume. How can I prevent this, so that ducking continues?

Code example, see lines 9 and 29:

import UIKit
import AVFoundation

class ViewController: UIViewController {
    override func viewDidLoad() {
        super.viewDidLoad()
        try! AVAudioSession.sharedInstance().setCategory(.playback, mode: .spokenAudio, options: [.duckOthers, .interruptSpokenAudioAndMixWithOthers])
        try! AVAudioSession.sharedInstance().setActive(true)
        // ducking starts here, as expected
        DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
            self.beginVideoWritingSession()
        }
    }

    func beginVideoWritingSession(){
        let videoSettings:[String: Any] = [AVVideoCodecKey: AVVideoCodecType.h264,
                                           AVVideoWidthKey: 200,
                                           AVVideoHeightKey: 200]
        let path = NSSearchPathForDirectoriesInDomains(.cachesDirectory, .userDomainMask, true)[0] + "/video"
        let fileURL = URL(fileURLWithPath: path)
        do {
            try FileManager.default.removeItem(at: fileURL)
        } catch {}
        let  assetWriter = try! AVAssetWriter(url: fileURL, fileType: AVFileType.mov)
        let writeInput = AVAssetWriterInput(mediaType: AVMediaType.video, outputSettings: videoSettings)
        assert(assetWriter.canAdd(writeInput))
        assetWriter.add(writeInput)
        assert(assetWriter.startWriting())
        // ducking stops here unexpectedly
    }
}

AVAudioSession ducking stops when AVAssetWriter.startWriting is called