It just playing a simple sound (A note).

var currentPhase = Float(), engine = AVAudioEngine()

func startStop()
{
    engine.isRunning ? engine.stop() : try? engine.start()
}

    override func viewDidLoad()
    {
        let twoPi       = 2 * Float.pi, frequency = Float(432.0), amplitude = Float(0.5)
        let output      = engine.outputNode, outputFormat = output.inputFormat(forBus: 0)
        let sampleRate  = Float(outputFormat.sampleRate)
        let inputFormat = AVAudioFormat(commonFormat: outputFormat.commonFormat, sampleRate: outputFormat.sampleRate, channels: 1, interleaved: outputFormat.isInterleaved)
        
        let srcNode = AVAudioSourceNode(renderBlock: { silence, timestamp, frameCount, audioBufferList -> OSStatus in
            
            for frame in 0..<Int(frameCount)
            {
                audioBufferList.pointee.mBuffers.mData!.storeBytes(of: sin(self.currentPhase) * amplitude, toByteOffset: frame * MemoryLayout<Float>.size, as: Float.self)
                
                self.currentPhase += twoPi / sampleRate * frequency
                self.currentPhase += self.currentPhase >= twoPi ? -twoPi : twoPi
            }
            return noErr
        })
        
        engine.attach(srcNode)
        engine.connect(srcNode, to: output, format: inputFormat)
        engine.prepare()
    }

Here is the message on Xcode :

com.apple.audio.IOThread.client (15): EXC_BREAKPOINT (code=1, subcode=0x104e249ec)