Swift 2.2, обязательно импортируйте MediaPlayer
private func setupVolumeListener()
{
let frameView:CGRect = CGRectMake(0, 0, 0, 0)
let volumeView = MPVolumeView(frame: frameView)
//self.window?.addSubview(volumeView) //use in app delegate
self.view.addSubview(volumeView) //use in a view controller
NSNotificationCenter.defaultCenter().addObserver(self, selector: #selector(volumeChanged(_:)), name: "AVSystemController_SystemVolumeDidChangeNotification", object: nil)
}//eom
func volumeChanged(notification:NSNotification)
{
let volume = notification.userInfo!["AVSystemController_AudioVolumeNotificationParameter"]
let category = notification.userInfo!["AVSystemController_AudioCategoryNotificationParameter"]
let reason = notification.userInfo!["AVSystemController_AudioVolumeChangeReasonNotificationParameter"]
print("volume: \(volume!)")
print("category: \(category!)")
print("reason: \(reason!)")
print("\n")
}//eom