How to record audio from the mic while appending a modified sampleBuffer image to the AVAssetWriter

Question

This is an extension to an unanswered question I had before: AVCaptureSession is not recording audio from the mic in Swift

I'm very unclear how I can write both video that is being modified in real time and audio that is being recorded from the microphone. I've been searching for months and I have found nothing. What seems to set my problem apart from others is that I am getting an image buffer from sampleBuffer in the captureOutput function, turning that into an image, modifying it, and then writing it back to a AVAssetWriterInputPixelBufferAdaptor, instead of recording everything from the output as normal video. From here, I have no idea what to do to get audio from sampleBuffer, or if that's even the correct approach, although I've seen others get an AudioBufferList from captureOutput.

Minimally, this is what I have in my main class:

class CaptureVC: UIViewController, AVCapturePhotoCaptureDelegate, AVCaptureVideoDataOutputSampleBufferDelegate, UIImagePickerControllerDelegate, UINavigationControllerDelegate,UIPickerViewDataSource,UIPickerViewDelegate {
    var captureSession: AVCaptureSession?
    var stillImageOutput: AVCapturePhotoOutput?
    var videoPreviewLayer: AVCaptureVideoPreviewLayer?
    let videoOutput = AVCaptureVideoDataOutput()
    let audioOutput = AVCaptureAudioDataOutput()

    var assetWriter: AVAssetWriter?
    var assetWriterPixelBufferInput: AVAssetWriterInputPixelBufferAdaptor?
    var assetWriterAudioInput: AVAssetWriterInput?
    var currentSampleTime: CMTime?
    var currentVideoDimensions: CMVideoDimensions?
    var videoIsRecording = false

    override func viewDidLoad() {
        super.viewDidLoad()

    let backCamera = AVCaptureDevice.default(for:AVMediaType.video)
    let microphone = AVCaptureDevice.default(.builtInMicrophone, for: AVMediaType.audio, position: .unspecified)

    var error: NSError?
    var videoInput: AVCaptureDeviceInput!
    var micInput: AVCaptureDeviceInput!
    do {
        videoInput = try AVCaptureDeviceInput(device: backCamera!)
        micInput = try AVCaptureDeviceInput(device: microphone!)
    } catch let error1 as NSError {
        error = error1
        videoInput = nil
        micInput = nil
        print(error!.localizedDescription)
    }

    if error == nil &&
        captureSession!.canAddInput(videoInput) &&
        captureSession!.canAddInput(micInput){

        captureSession!.addInput(videoInput)
        captureSession!.addInput(micInput)

        stillImageOutput = AVCapturePhotoOutput()


        if captureSession!.canAddOutput(stillImageOutput!) {
            captureSession!.addOutput(stillImageOutput!)

            let q = DispatchQueue(label: "sample buffer delegate", qos: .default)

            videoOutput.setSampleBufferDelegate(self, queue: q)

            if captureSession!.canAddOutput(videoOutput){
                captureSession!.addOutput(videoOutput)
            }

            audioOutput.setSampleBufferDelegate(self as? AVCaptureAudioDataOutputSampleBufferDelegate, queue: q)

            if captureSession!.canAddOutput(audioOutput){
                captureSession!.addOutput(audioOutput)
            }

            captureSession!.startRunning()

        }

    }

}

My class that creates the asset writer

func createWriter() {
    self.checkForAndDeleteFile()

    do {
        assetWriter = try AVAssetWriter(outputURL: movieURL() as URL, fileType: AVFileType.mov)
    } catch let error as NSError {
        print(error.localizedDescription)
        return
    }

    let videoSettings = [
        AVVideoCodecKey : AVVideoCodecType.h264,
        AVVideoWidthKey : Int(currentVideoDimensions!.height), // note: these are swapped because of REASONS
        AVVideoHeightKey : Int(currentVideoDimensions!.width)
    ] as [String : Any]

    let assetWriterVideoInput = AVAssetWriterInput(mediaType: AVMediaType.video, outputSettings: videoSettings)
    assetWriterVideoInput.expectsMediaDataInRealTime = true

    let sourcePixelBufferAttributesDictionary = [
        String(kCVPixelBufferPixelFormatTypeKey) : Int(kCVPixelFormatType_32BGRA),
        String(kCVPixelBufferWidthKey) : Int(currentVideoDimensions!.width),
        String(kCVPixelBufferHeightKey) : Int(currentVideoDimensions!.height),
        String(kCVPixelFormatOpenGLESCompatibility) : kCFBooleanTrue
        ] as [String : Any]

    assetWriterPixelBufferInput = AVAssetWriterInputPixelBufferAdaptor(assetWriterInput: assetWriterVideoInput,
                                                                       sourcePixelBufferAttributes: sourcePixelBufferAttributesDictionary)


    var channelLayout = AudioChannelLayout()
    memset(&channelLayout, 0, MemoryLayout<AudioChannelLayout>.size);
    channelLayout.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo;

    let audioSettings: [String: Any] = [AVFormatIDKey: kAudioFormatMPEG4AAC,
                                        AVSampleRateKey: 44100,
                                        AVNumberOfChannelsKey: 2]

    assetWriterAudioInput = AVAssetWriterInput(mediaType: AVMediaType.audio, outputSettings: audioSettings)
    assetWriterAudioInput?.expectsMediaDataInRealTime = true

    if assetWriter!.canAdd(assetWriterVideoInput) {
        assetWriter!.add(assetWriterVideoInput)
    } else {
        print("cannot add \(assetWriterVideoInput)")
    }

    if assetWriter!.canAdd(assetWriterAudioInput!) {
        assetWriter!.add(assetWriterAudioInput!)
    } else {
        print("cannot add \(String(describing: assetWriterAudioInput))")
    }

}

When I record I create the writer, start writing and start the session:

        videoIsRecording = true

        createWriter()
        assetWriter?.startWriting()
        assetWriter?.startSession(atSourceTime: currentSampleTime!)

When I stop the recording, it goes to another view to display the video:

        assetWriter?.finishWriting(completionHandler: {[unowned self] () -> Void in

            let firstAsset = AVURLAsset(url: self.movieURL() as URL)

            guard let exporter = AVAssetExportSession(asset: firstAsset, presetName: AVAssetExportPresetHighestQuality) else { return }



            guard let vidComp = self.getVideoComposition(asset: firstAsset,
                                                         videoSize: CGSize(width:1280,
                                                                           height:720)) else {
                print("Unable to create video composition")
                return
            }
            print(vidComp.instructions)
            exporter.videoComposition = vidComp
            exporter.outputURL = self.movieURL() as URL
            exporter.outputFileType = AVFileType.mov

            exporter.exportAsynchronously() {
                DispatchQueue.main.async(){

                    self.activityTextStatus.text = ""

                    fileURLSenderVal = self.movieURL() as URL

                    let manageCaptureVC = self.storyboard?.instantiateViewController(withIdentifier: "ManageCaptureVC") as! ManageCaptureVC

                    manageCaptureVC.fileURL = fileURLSenderVal
                    manageCaptureVC.imageCaptureMode = ManageCaptureVC.imageCaptureModes(rawValue: self.imageCaptureMode.rawValue)!
                    manageCaptureVC.delegate = self

                    self.present(manageCaptureVC, animated: true, completion: nil)

                }
            }


        })

But this is where I'm stuck - where and how do I record my microphone's input??

// live output from camera
func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection){

    if(captureOutput){

        let pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer)
        let cameraImage = CIImage(cvPixelBuffer: pixelBuffer!)

        var orientation = UIImageOrientation.right

        if(isFrontCamera){

            orientation = UIImageOrientation.leftMirrored

        }


        image = UIImage(ciImage: cameraImage)

        if let ciImage = image?.ciImage {

            image = applyFilterAndReturnImage(ciImage: ciImage, orientation: orientation, currentCameraRes:currentCameraRes!)

            let formatDescription = CMSampleBufferGetFormatDescription(sampleBuffer)!
            self.currentVideoDimensions = CMVideoFormatDescriptionGetDimensions(formatDescription)
            self.currentSampleTime = CMSampleBufferGetOutputPresentationTimeStamp(sampleBuffer)

            if(videoIsRecording && self.assetWriterPixelBufferInput?.assetWriterInput.isReadyForMoreMediaData == true){

                let filteredBuffer = buffer(from: image!)

                let success = self.assetWriterPixelBufferInput?.append(filteredBuffer!, withPresentationTime: self.currentSampleTime!)



                if success == false {
                    print("Pixel Buffer failed")
                }

            }


        DispatchQueue.main.async(){
            imageView!.image = image
        }

    }

}

Again, I've been at this on and off for months - No documents I can find help. Thank you