该问题的解决方法是使用FFMpeg库，将.m4a文件转换为.wav格式。以下是代码示例：

import AudioKit
import AVFoundation

// M4A文件路径
let m4aFilePath = Bundle.main.path(forResource: "input", ofType: "m4a")!
let m4aURL = URL(fileURLWithPath: m4aFilePath)

// WAV文件存储路径
let documentsPath = NSSearchPathForDirectoriesInDomains(.documentDirectory, .userDomainMask, true)[0]
let wavFilePath = "\(documentsPath)/output.wav"
let wavURL = URL(fileURLWithPath: wavFilePath)

let asset = AVURLAsset(url: m4aURL)
let exporter = AVAssetExportSession(asset: asset, presetName: AVAssetExportPresetAppleM4A)

exporter?.outputFileType = AVFileType.wav
exporter?.outputURL = wavURL

// 开始转换
exporter?.exportAsynchronously(completionHandler: {
    guard let status = exporter?.status else {
        return
    }
    
    if status == .completed {
        // 转换成功
        let wavFile = try! AKAudioFile(forReading: wavURL)
        let player = try! AKAudioPlayer(file: wavFile)
        let mixer = AKMixer(player)
        AudioKit.output = mixer
        try! AudioKit.start()
        player.play()
    } else {
        // 转换失败
        print("转换失败")
    }
})