以下是一个使用AVAudioEngine进行多声道映射的示例代码：

import AVFoundation

// 创建AVAudioEngine实例
let engine = AVAudioEngine()

// 创建多声道音频文件的URL
let fileURL = Bundle.main.url(forResource: "multichannel_audio", withExtension: "wav")

// 创建AVAudioFile实例
let audioFile = try AVAudioFile(forReading: fileURL!)

// 创建AVAudioPlayerNode实例
let playerNode = AVAudioPlayerNode()

// 连接AVAudioPlayerNode到AVAudioEngine的输出节点
engine.attach(playerNode)
engine.connect(playerNode, to: engine.mainMixerNode, format: nil)

// 启动AVAudioEngine
try engine.start()

// 播放音频文件
playerNode.scheduleFile(audioFile, at: nil, completionHandler: nil)
playerNode.play()

// 创建一个自定义的音频输出节点
let customOutputNode = AVAudioSinkNode { (audioBuffer, audioTime) in
    // 遍历每个声道的音频数据
    for channel in 0..