AVAudioplayer: volume drops after a couple of seconds

I implemented an audio player on iOS 10.2.1 with Xcode Version 8.2.1 (8C1002) on Mac OS Sierra 10.12.3

When the song starts playing, the volume will decrease slightly after a couple of seconds.

Has anyone experienced such an effect?

 do {
     myAudioPlayer = try AVAudioPlayer(contentsOf: url!)
     myAudioPlayer.prepareToPlay()
     myAudioPlayer.volume = 1.0
 } catch  {
     print(error)
 }