volumeChangeNotification.swift

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
  NotificationCenter.default.addObserver(self, selector: Selector(("volumeDidChange:")), name: NSNotification.Name(rawValue: "AVSystemController_SystemVolumeDidChangeNotification"), object: nil)
  return true
}

 func volumeDidChange(notification: NSNotification) {
   let volume = notification.userInfo!["AVSystemController_AudioVolumeNotificationParameter"] as! Float
        
   // Volume at your service

 }