When using AVFoundation to play sound, there is sound at simulator, but no sound on device

Question

As I was trying to play sound in my timer app, I use AVFoundation in the code. While I am able to hear the sound in simulator, but not in device. I wonder if I need to set something for the app to play sound in device?

Please find the below code for more information.

import AVFoundation
class timer {
  var player: AVAudioPlayer = AVAudioPlayer()
     // in viewdidload
    let squishPath = NSBundle.mainBundle().pathForResource("ding", ofType: "wav")
    let squishURL = NSURL(fileURLWithPath: squishPath!)

    do {
        try squishPlayer = AVAudioPlayer(contentsOfURL: squishURL)
        squishPlayer.prepareToPlay()
    } catch let err as NSError {
        print(err.debugDescription)
    }
    squishPlayer.numberOfLoops = 0
}

I checked the file name is exactly and it is in the copy bounce resources.

I found the solution is to add the following to app delegate.

func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
    let session = AVAudioSession.sharedInstance()

    do {
        try session.setCategory(AVAudioSessionCategoryPlayback)
        try session.setActive(true)
    } catch let error as NSError {
        print("AVAudioSession configuration error: \(error.localizedDescription)")
    }


    return true
}

And this solves the problem of no sound on device, but strange thing is even if I didn't add these to app delegate, there is still sound in simulator.