Swift Playground shows how to play multiple wav files on top of background audio file, using AVAudioPlayerNode, AVAudioMixerNode, AVAudioEngine

AudioPlayground.swift

import AVFoundation

import UIKit
import PlaygroundSupport

PlaygroundPage.current.needsIndefiniteExecution = true

class AudioPlayer {
    var backgroundAudioFile:AVAudioFile
    var topAudioFiles: [AVAudioFile] = []
    var engine:AVAudioEngine
    var backgroundAudioNode: AVAudioPlayerNode
    var topAudioAudioNodes = [AVAudioPlayerNode]()
    var mixer: AVAudioMixerNode
    var timer: Timer!
    var urls: [URL] = []
    
    init (_ url: URL, urls: [URL] = []) {
        backgroundAudioFile = try! AVAudioFile(forReading: url)
        topAudioFiles = urls.map { try! AVAudioFile(forReading: $0) }
        
        engine = AVAudioEngine()
        mixer = AVAudioMixerNode()
        
        engine.attach(mixer)
        engine.connect(mixer, to: engine.outputNode, format: nil)
        self.urls = urls
        backgroundAudioNode = AVAudioPlayerNode()
        for _ in topAudioFiles {
            topAudioAudioNodes += [AVAudioPlayerNode()]
        }
    }
    
    func start() {
        engine.attach(backgroundAudioNode)
        engine.connect(backgroundAudioNode, to: mixer, format: nil)
        backgroundAudioNode.scheduleFile(backgroundAudioFile, at: nil, completionHandler: nil)
        
        try! engine.start()
        backgroundAudioNode.play()
        
        for node in topAudioAudioNodes {
            engine.attach(node)
            engine.connect(node, to: mixer, format: nil)
        }
        try! engine.start()
        // simulate rescheduling files played on top of background audio
        DispatchQueue.global().async { [unowned self] in
            for i in 0..<1000 {
                sleep(2)
                let index = i % self.topAudioAudioNodes.count
                let node = self.topAudioAudioNodes[index]
                node.scheduleFile(self.topAudioFiles[index], at: nil, completionHandler: nil)
                node.play()
            }
        }
    }
}

let bundle = Bundle.main
let beepLow = bundle.url(forResource: "beeplow", withExtension: "wav")!
let beepMid = bundle.url(forResource: "beepmid", withExtension: "wav")!
let backgroundAudio = bundle.url(forResource: "backgroundAudio", withExtension: "wav")!
let audioPlayer = AudioPlayer(backgroundAudio, urls: [beepLow, beepMid])
audioPlayer.start()