Phonon problem in Windows 7 with code compiled in XP

Question

I have some code compiled using Visual Studio 2010 (C++), with Qt for the GUI and Phonon to show some videos.

I compile and run the code in a Windows XP machine and everything works fine. The videos and controls are shown correctly, and the same in other Windows XP machines. But at the moment that I try to use a Windows 7 machine, the video is not shown. Phonon controls are loaded, I can adjust the size of the Phonon VideoWidget but nothing is shown. I am using the SetFile method to indicate the file location and I have also tried (of course, with no luck) with setCurrentSource in the MediaObject (it works, though, in Windows XP).

I have tried different locations of files (absolute/relative) but nothing seems to work. Also, in XP when a video is not found an alert message is shown while in Windows 7 there is no error message, but also no video is shown. If I try to open the videos with other players, they work fine (I tried also with videos with different formats).

Any idea on what can be wrong? Is there something that I am missing with Phonon and Windows 7? Thanks!