Error: "Sink Format Changed" while using qtMultimedia

I was trying to program a really simple audio player using QtMultimedia in qt5. I used a code like this:

AudioPlayer::AudioPlayer(QWidget* parent): QWidget(parent)
{
   //some code

   audioPlayer = new QMediaPlayer;
   audioPlayer -> setMedia(QUrl::fromLocalFile("/home/davide/Music.mp3")); 
   audioPlayer -> setVolume(50);
   audioPlayer -> play();
   //some other code
}

This code compiles fine, but when I run it I get the message: <> and the audio doesn't play...

I tried playing a wav audio file, and it works fine so I think the problem is in the mp3 format.

Thanks in advance and sorry for my bad english.