Issues with saving `AVAudioFile` - playable, no sound

Hello, I am trying to use AVAudioFile to save audio buffer to .wav file. The buffer is of type [Float]. Currently I am able to successfully create the .wav files and even play them, but they are blank - I cannot hear any sound.

private func saveAudioFile(using buffer: [Float]) {
        let fileUrl = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!.appendingPathComponent("\(UUID().uuidString).wav")

        let fileSettings = [
            AVFormatIDKey: Int(kAudioFormatLinearPCM),
            AVSampleRateKey: 15600,
            AVNumberOfChannelsKey: 1,
            AVEncoderAudioQualityKey: AVAudioQuality.high.rawValue
        ]

        guard let file = try? AVAudioFile(forWriting: fileUrl, settings: fileSettings, commonFormat: .pcmFormatInt16, interleaved: true) else {
            print("Cannot create AudioFile")
            return
        }
        guard let bufferFormat = AVAudioFormat(settings: settings) else {
            print("Cannot create buffer format")
            return
        }

        guard let outputBuffer = AVAudioPCMBuffer(pcmFormat: bufferFormat, frameCapacity: AVAudioFrameCount(buffer.count)) else {
            print("Cannot create output buffer")
            return
        }
        for i in 0..<buffer.count {
            outputBuffer.int16ChannelData!.pointee[i] = Int16(buffer[i])
        }
        outputBuffer.frameLength = AVAudioFrameCount(buffer.count)

        do {
            try file.write(from: outputBuffer)
        } catch {
            print(error.localizedDescription)
            print("Write to file failed")
        }   
    }

Where should I be looking first for the problem? Is it format issue? I am getting the data from the microphone with the AVAudioEngine. Its format is created like this:

let outputFormat = AVAudioFormat(commonFormat: .pcmFormatInt16, sampleRate: Double(15600), channels: 1, interleaved: true)!

And here is the installTap implementation with the buffer callback:

input.installTap(onBus: 0, bufferSize: AVAudioFrameCount(sampleRate*2), format: inputFormat) { (incomingBuffer, time) in
            DispatchQueue.global(qos: .background).async {
                let pcmBuffer = AVAudioPCMBuffer(pcmFormat: outputFormat, frameCapacity: AVAudioFrameCount(outputFormat.sampleRate * 2.0))
                var error: NSError? = nil

                let inputBlock: AVAudioConverterInputBlock = { inNumPackets, outStatus in
                    outStatus.pointee = AVAudioConverterInputStatus.haveData
                    return incomingBuffer
                }

                formatConverter.convert(to: pcmBuffer!, error: &error, withInputFrom: inputBlock)

                if error != nil {
                    print(error!.localizedDescription)
                }
                else if let pcmBuffer = pcmBuffer, let channelData = pcmBuffer.int16ChannelData {

                    let channelDataPointer = channelData.pointee
                    self.buffer = stride(from: 0, to: self.windowLengthSamples, by: 1).map { Float(channelDataPointer[$0]) / 32768.0 }
                    onBufferUpdated(self.buffer)
                }
            }
        }

The onBufferUpdated is the block that provides [Float] for the saveAudioFile method above.

I have tried some experiements with different output formats, but that ended up with unplayable audio files.