Audio from a file generated from an AVAudioEngine bus tap has different speed than the one played on device output

Question

I am generating audio with an AVAudioEngine. I am then installing a tap on the mainMixerNode output of the engine which provides a AVAudioPCMBuffer which is then written into an MPEG4ACC AVAudioFile. The issue I have is that the audio played on the resulting .m4a file is slower in speed than the one that you can hear on the device output itself (speakers, headphones). This is the code I am implementing:

// Audio Format
let audioFormat = AVAudioFormat(standardFormatWithSampleRate: 44100, channels: 2)

// Engine
var engine = AVAudioEngine()

// Two AVAudioNodes are hooked up to the AVAudioEngine
engine.connect(myAVAudioNode0, to: engine.mainMixerNode, format: audioFormat)
engine.connect(myAVAudioNode1, to: engine.mainMixerNode, format: audioFormat)

// Function to Write Audio to a File
func writeAudioToFile() {
        // File to write
        let documentsDirectory = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask).first!
        let audioURL = documentsDirectory.appendingPathComponent("share.m4a")

        // Format parameters
        let sampleRate = Int(audioFormat!.sampleRate)
        let channels = Int(audioFormat!.channelCount)

        // Audio File settings
        let settings = [
            AVFormatIDKey: Int(kAudioFormatMPEG4AAC),
            AVSampleRateKey: Int(audioFormat!.sampleRate),
            AVNumberOfChannelsKey: Int(audioFormat!.channelCount),
            AVEncoderAudioQualityKey: AVAudioQuality.max.rawValue
        ]

        // Audio File
        var audioFile = AVAudioFile()
        do {
            audioFile = try AVAudioFile(forWriting: audioURL, settings: settings, commonFormat: .pcmFormatFloat32, interleaved: false)
        }
        catch {
            print ("Failed to open Audio File For Writing: \(error.localizedDescription)")
        }

        // Install Tap on mainMixer
        // Write into buffer and then write buffer into AAC file
        engine.mainMixerNode.installTap(onBus: 0, bufferSize: 8192, format: nil, block: { (pcmBuffer, when) in
            do {
                try audioFile.write(from: pcmBuffer)
            }
            catch {
                print("Failed to write Audio File: \(error.localizedDescription)")
            }
        })
}

Answer 1

I printed out the AVAudioPCMBuffer format and found that it was defaulting to a sample rate of 48000Hz. But this is not always the case. First, determine the audio session sample rate and use that sample rate for the audio format:

let audioSession = AVAudioSession.sharedInstance()
let sampleRate = audioSession.sampleRate
let audioFormat = AVAudioFormat(standardFormatWithSampleRate: sampleRate, channels: 2)