song beep

import AVFoundation
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

class AudioPlayer {
    var topAudioFiles: [AVAudioFile] = []
    var engine:AVAudioEngine
    var backgroundAudioNode: AVAudioPlayerNode
    var backgroundAudioFile: AVAudioFile
    var topAudioAudioNodes = [AVAudioPlayerNode]()
    var mixer: AVAudioMixerNode
    var timer: Timer!
    var urls: [URL] = []
    var player: AVPlayer!
    var times = [NSValue]()
    var delays = [UInt64]()
    
    init (_ url: URL, urls: [URL] = []) {
        self.urls = urls
        topAudioFiles = urls.map { try! AVAudioFile(forReading: $0) }
        backgroundAudioFile = try! AVAudioFile(forReading: url)
        player = AVPlayer(url: url)
        engine = AVAudioEngine()
        mixer = AVAudioMixerNode()
        
        engine.attach(mixer)
        engine.connect(mixer, to: engine.outputNode, format: nil)
        backgroundAudioNode = AVAudioPlayerNode()
        
        initTopAudioNodes()
        try! engine.start()
    }
    
   
    
    func initTopAudioNodes() {
        for _ in topAudioFiles {
            topAudioAudioNodes += [AVAudioPlayerNode()]
        }
        for node in topAudioAudioNodes {
            engine.attach(node)
            engine.connect(node, to: mixer, format: nil)
        }
    }
    
    func playWithAudioPlayerAndNodes() {
        player.play()
        var i = 1
        
        player.addBoundaryTimeObserver(forTimes: times, queue: nil) {
            let index = i % self.topAudioAudioNodes.count
            let node = self.topAudioAudioNodes[index]
            node.scheduleFile(self.topAudioFiles[index], at: nil, completionHandler: nil)
            node.play()
            i += 1
        }
    }

}//end class

let bundle = Bundle.main
let beepLow = bundle.url(forResource: "1", withExtension: "wav")!
let beepMid = bundle.url(forResource: "2", withExtension: "wav")!
  let backgroundAudio = URL(string: "https://www.soundhelix.com/examples/mp3/SoundHelix-Song-1.mp3")!
  //even if I try to use a local file, it will still output error. For example  let backgroundAudio = bundle.url(forResource: "2", withExtension: "wav")!

let audioPlayer = AudioPlayer(backgroundAudio, urls: [beepLow, beepMid])

// second approach using AVPlayer and boundary time observer:
audioPlayer.playWithAudioPlayerAndNodes()