title: avplayer has no sounds when play video
body: <p>here is all my codes for play a mp4,the mp4 is playing but no sounds out </p>
  
  <pre><code>import UIKit
  
  import AVFoundation
  class ViewController: UIViewController {
  
  @IBOutlet weak var containerView: UIView!
  
  var playerLayer:AVPlayerLayer!
  var player:AVPlayer!
  override func viewDidLoad() {
      super.viewDidLoad()
      // Do any additional setup after loading the view, typically from a nib.
      let path = NSBundle.mainBundle().pathForResource("huoguo", ofType: "mp4")
      let url = NSURL(fileURLWithPath: path!)
      let asset = AVAsset(URL: url)
      let playerItem = AVPlayerItem(asset: asset)
      self.player = AVPlayer(playerItem: playerItem)
      self.playerLayer = AVPlayerLayer(player: player)
      self.playerLayer.frame = CGRectMake(0, 0, UIScreen.mainScreen().bounds.width, UIScreen.mainScreen().bounds.width * 9 / 16)
      self.containerView.layer.addSublayer(playerLayer)
      //self.playerLayer.player?.volume = 1
      self.playerLayer.player?.play()
  }
  }
  </code></pre>
  
  <p>i'm sure it has sounds,because i can hear when play the mp4 using VLC,  did i miss something ?</p>