I'm trying to use AVCaptureSession and AVAssetWriter to convert video and audio from an iPhone's camera and microphone into a fragmented video file in AppleHLS format. Below is part of the code. It seems that the capture is successful, and I have confirmed that the data received with captureOutput() can be appended to videoWriterIput and audioWriterInput using append(). When executing audioWriterInput!.append(sampleBuffer), sampleBuffer has the following value, and it looks like the audio data has been passed to AssetWriter. sampleBuffer.duration : CMTime(value: 941, timescale: 44100, flags: __C.CMTimeFlags(rawValue: 1), epoch: 0) sampleBuffer.totalSampleSize : 1882 However, the final output init.mp4 and *.m4s do not contain Audio. (The video can be played without any problems.) Could you please tell me any problems or hints as to why Audio is not included? /// Capture Session let captureSession = AVCaptureSession() /// Capture Input var videoDevice: AVCaptureDevice? var audioDevice: AVCaptureDevice? /// Configure and Start Capture Session func startCapture() { // Start Configuration captureSession.beginConfiguration() // Setup Input Video videoDevice = self.defaultCamera(cameraSide: cameraSide) videoDevice!.activeVideoMinFrameDuration = CMTimeMake(value: 1, timescale: 30) let videoInput = try AVCaptureDeviceInput(device: videoDevice!) as AVCaptureDeviceInput captureSession.addInput(videoInput) // Setup Input Audio audioDevice = AVCaptureDevice.default(for: AVMediaType.audio) let audioInput = try AVCaptureDeviceInput(device: audioDevice!) as AVCaptureDeviceInput captureSession.addInput(audioInput) // Setup Output Video let videoDataOutput = AVCaptureVideoDataOutput() videoDataOutput.setSampleBufferDelegate(self, queue: recordingQueue) videoDataOutput.alwaysDiscardsLateVideoFrames = true videoDataOutput.videoSettings = [ kCVPixelBufferPixelFormatTypeKey: Int(kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange)] as [String : Any] captureSession.addOutput(videoDataOutput) // Setup Output Audio let audioDataOutput = AVCaptureAudioDataOutput() audioDataOutput.setSampleBufferDelegate(self, queue: recordingQueue) captureSession.addOutput(audioDataOutput) //End Configuration captureSession.commitConfiguration() // Start Capture captureSession.startRunning() } private let assetWriter: AVAssetWriter? private let startTimeOffset: CMTime private var audioWriterInput: AVAssetWriterInput? private let videoWriterInput: AVAssetWriterInput? func captureOutput(_ output: AVCaptureOutput, didOutput sampleBuffer: CMSampleBuffer, from connection: AVCaptureConnection) { if assetWriter == nil { // AssetWriter assetWriter = AVAssetWriter(contentType: UTType(AVFileType.mp4.rawValue)!) self.startTimeOffset = CMTime(value: 1, timescale: 1) // Setup Input of Audio. let audioCompressionSettings: [String: Any] = [ AVFormatIDKey: kAudioFormatMPEG4AAC, AVSampleRateKey: 44_100, AVNumberOfChannelsKey: 1, AVEncoderBitRateKey: 128_000 ] audioWriterInput = AVAssetWriterInput(mediaType: .audio, outputSettings: audioCompressionSettings) audioWriterInput!.expectsMediaDataInRealTime = true assetWriter.add(audioWriterInput!) // Setup Input of Video. let videoCompressionSettings: [String: Any] = [ AVVideoCodecKey: AVVideoCodecType.h264 ] let videoCompressionSettings: [String: Any] = [ AVVideoCodecKey: AVVideoCodecType.h264, AVVideoWidthKey: 1280, AVVideoHeightKey: 720, AVVideoCompressionPropertiesKey: [ kVTCompressionPropertyKey_AverageBitRate: 1_024_000, kVTCompressionPropertyKey_ProfileLevel: kVTProfileLevel_H264_Baseline_AutoLevel ] ] videoWriterInput = AVAssetWriterInput(mediaType: .video, outputSettings: videoCompressionSettings) videoWriterInput!.expectsMediaDataInRealTime = true pixelBuffer = AVAssetWriterInputPixelBufferAdaptor(assetWriterInput: videoWriterInput!, sourcePixelBufferAttributes: [kCVPixelBufferPixelFormatTypeKey as String: Int(kCVPixelFormatType_32BGRA)]) assetWriter.add(videoWriterInput!) // Configure the asset writer for writing data in fragmented MPEG-4 format. assetWriter.outputFileTypeProfile = AVFileTypeProfile.mpeg4AppleHLS assetWriter.preferredOutputSegmentInterval = CMTime(seconds: 1.0, preferredTimescale: 1) assetWriter.initialSegmentStartTime = startTimeOffset assetWriter.delegate = self // start AssetWriiter startTime = CMSampleBufferGetPresentationTimeStamp(sampleBuffer) assetWriter.startWriting() assetWriter.startSession(atSourceTime: startTime) } let isVideo = output is AVCaptureVideoDataOutput if isVideo { if videoWriterInput.isReadyForMoreMediaData { videoWriterInput!.append(sampleBuffer) } }else{ if audioWriterInput!.isReadyForMoreMediaData { audioWriterInput!.append(sampleBuffer) } } } func assetWriter(_ writer: AVAssetWriter, didOutputSegmentData segmentData: Data, segmentType: AVAssetSegmentType, segmentReport: AVAssetSegmentReport?) { : : }