Swift 4 Using KVO to listen to volume changes

I just updated to Swift 4 and Xcode 9 and got a (swiftlint) warning for the following code telling me that I should use KVO now:

Warning:

(Block Based KVO Violation: Prefer the new block based KVO API with keypaths when using Swift 3.2 or later. (block_based_kvo))

The old code:

override func observeValue(forKeyPath keyPath: String?,
                           of object: Any?,
                           change: [NSKeyValueChangeKey : Any]?,
                           context: UnsafeMutableRawPointer?) {
    if keyPath == "outputVolume"{
        guard let newKey = change?[NSKeyValueChangeKey.newKey] as? NSNumber else {
            fatalError("Could not unwrap optional content of new key")
        }

        let volume = newKey.floatValue

        print("volume " + volume.description)
    }
}

My attempt to fix:

let audioSession = AVAudioSession.sharedInstance()
    audioSession.observe(\.outputVolume) { (av, change) in
        print("volume \(av.outputVolume)")
}

Apple claims here that most of the properties should be dynamic (I know that this is AVPlayer and not AVAudioSession). I looked it up but couldn't find any dynamic statements inside AVPlayer properties and was wondering how that could possibly work (If I'm not mistaken those are required for KVO to work).

EDIT:

I'm not certain if it doesn't trigger because it simply doesn't work or if it's due to what I try to archive. In general I'll want to get notified on volume changes triggered by pushing the hardware-volume-rockers.

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