Player.swift

import UIKit
import AVFoundation
import PlaygroundSupport

class Player: NSObject {

  var player: AVQueuePlayer!
  var playerItem: AVPlayerItem!
  var playerContext = 0

  override init() {
    super.init()
    let itemURL = URL(string: "https://ia802704.us.archive.org/6/items/AlannSteenHostageReleaseDocumentary1991/AlannSteen.mp3")!
    playerItem = AVPlayerItem(url: itemURL)
    playerItem.addObserver(self, forKeyPath: "loadedTimeRanges", options: [.initial, .new], context: &playerContext)
  }
  
  override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
    if let playerItem =  object as? AVPlayerItem, let first = playerItem.loadedTimeRanges.first {
      let loaded = first.timeRangeValue.duration.value.hashValue / first.timeRangeValue.duration.timescale.hashValue
      let total = playerItem.duration.value.hashValue / playerItem.duration.timescale.hashValue
      print("\((loaded*100)/total)%")
    }
  }
  
  func play() {
    player = AVQueuePlayer(playerItem: playerItem)
    player.play()
  }
  
  deinit {
    playerItem.removeObserver(self, forKeyPath: "loadedTimeRanges")
  }
  
}

var player = Player()
player.play()

PlaygroundPage.current.needsIndefiniteExecution = true