Understanding Mediaplayer Demo

[Phole]

I'm hoping someone can help me understand something. I'm trying to learn from Qt's media player demo but there's something I'm not quite getting. The volume is controlled by phonon's volume slider which, as I understand, directly controls the volume through phonon's audioOutput. If that's the case, why is there a setVolume function? Is that function necessary for the volume slider to work?

http://qt-project.org/doc/qt-4.7/demos-qmediaplayer-mediaplayer-cpp.html