Am unable to add an AVAudioMixerNode to downsize my recording

Am at the beginning of a voice recording app. I store incoming voice data into a buffer array, and write 50 of them to a file. The code works fine, Sample One.

However, I would like the recorded files to be smaller. So here I try to add an AVAudioMixer to downsize the sampling. But this code sample gives me two errors. Sample Two

The first error I get is when I call audioEngine.attach(downMixer). The debugger gives me nine of these errors:

throwing -10878

The second error is a crash when I try to write to audioFile. Of course they might all be related, so am looking to include the mixer successfully first.

But I do need help as I am just trying to piece these all together from tutorials, and when it comes to audio, I know less than anything else.

Sample One

//these two lines are in the init of the class that contains this function...
		node = audioEngine.inputNode
		recordingFormat = node.inputFormat(forBus: 0)


func startRecording() 
{
	audioBuffs = []
	x = -1
	node.installTap(onBus: 0, bufferSize: 8192, format: recordingFormat, block: {
		[self]
		(buffer, _) in
		x += 1
		audioBuffs.append(buffer)

		if x >= 50 {
			audioFile = makeFile(format: recordingFormat, index: fileCount)
			mainView?.setLabelText(tag: 3, text: "fileIndex = \(fileCount)")

			fileCount += 1
			for i in 0...49 {
				do {
					try audioFile!.write(from: audioBuffs[i]);
				} catch {
					mainView?.setLabelText(tag: 4, text: "write error")
					stopRecording()
				}
			}
			...cleanup buffer code
		}
	})
	

	audioEngine.prepare()
	do {
		try audioEngine.start()
	} catch let error { print ("oh catch \(error)") }
}

Sample Two

//these two lines are in the init of the class that contains this function
		node = audioEngine.inputNode
		recordingFormat = node.inputFormat(forBus: 0)

func startRecording() {
	audioBuffs = []
	x = -1

    // new code 
	let format16KHzMono = AVAudioFormat.init(commonFormat: AVAudioCommonFormat.pcmFormatInt16, sampleRate: 11025.0, channels: 1, interleaved: true)
	let downMixer = AVAudioMixerNode()
	audioEngine.attach(downMixer)

    // installTap on the mixer rather than the node
	downMixer.installTap(onBus: 0, bufferSize: 8192, format: format16KHzMono, block: {
		[self]
		(buffer, _) in
		x += 1
		audioBuffs.append(buffer)

		if x >= 50 {
            // use a different format in creating the audioFile
			audioFile = makeFile(format: format16KHzMono!, index: fileCount)
			mainView?.setLabelText(tag: 3, text: "fileIndex = \(fileCount)")

			fileCount += 1
			for i in 0...49 {
				do {
					try audioFile!.write(from: audioBuffs[i]);
				} catch {
					stopRecording()
				}
			}
			...cleanup buffers...
		}
	})
	
	let format = node.inputFormat(forBus: 0)

        // new code 
	audioEngine.connect(node, to: downMixer, format: format)//use default input format
	audioEngine.connect(downMixer, to: audioEngine.outputNode, format: format16KHzMono)//use new audio format
	downMixer.outputVolume = 0.0
	
	
	audioEngine.prepare()
	do {
		try audioEngine.start()
	} catch let error { print ("oh catch \(error)") }
}