ViewController.swift

// ...
import CoreData
import AVFoundation
// ...

class ViewController: UIViewController {
  
  // ...
  var coreDataContext:NSManagedObjectContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
  // ...
  
  func playPhraseSound(_ sentence:Sentence){
    let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "AudioCache")
    fetchRequest.predicate = NSPredicate(format: "remoteUrl = %@", sentence.soundUrl)
    
    do {
      let audioCaches = try self.coreDataContext.fetch(fetchRequest)
      var playerItem:AVPlayerItem
      
      if let audioCache = audioCaches.first as? AudioCache {
        // There is cache on CoreData
        playerItem = AVPlayerItem(asset: audioCache.data!.getAVAsset())
      } else {
        // There isn't cache on CoreData
        self.activityIndicatorView.startAnimating()
        let newAudioCache = AudioCache(context: self.coreDataContext)
        do{
          newAudioCache.remoteUrl = sentence.soundUrl
          guard let url = URL.init(string: sentence.soundUrl) else { return }
          newAudioCache.data = try! Data(contentsOf: url)
          try self.coreDataContext.save()
        } catch let err {
          print("error on making audio cache", err)
        }
        playerItem = AVPlayerItem(asset: newAudioCache.data!.getAVAsset())
      }
      
      player = AVPlayer.init(playerItem: playerItem)
    } catch let err {
      print("error on playing", err)
    }
    
    player?.play()
  }
  
  // ...
}

extension Data {
  func getAVAsset() -> AVAsset {
    let directory = NSTemporaryDirectory()
    let fileName = "\(NSUUID().uuidString).wav"
    let fullURL = NSURL.fileURL(withPathComponents: [directory, fileName])
    try! self.write(to: fullURL!)
    let asset = AVAsset(url: fullURL!)
    return asset
  }
}