Did some more testing today and created a minimal example:
1: create a new Qt 4 Gui Application (with Phonon support)
2: create a simple menu in the form editor
3: in mainwindow.cpp:
-
Qt Code: 
  1. #include <phonon/audiooutput.h>
To copy to clipboard, switch view to plain text mode 
- and in the MainWindow constructor:
Qt Code: 
  1. Phonon::AudioOutput(Phonon::MusicCategory, this);
To copy to clipboard, switch view to plain text mode 
I also tested on my laptop running windows xp professional and there the problem doesn't exist so I guess there is a problem with my installation.

As I said in my original post I installed Qt 4.4.3 from the lenny repositories
I have phonon 4.4.2 (also the latest in lenny repositories) and it's dev packages.

I also have Qt 4.6 compiled from source for a different project, but I explicitly set my project to use Qt 4.4.