Phonon can work with either VLC or GStreamer backend. Which one it uses by default when multiple are installed can be set by the user using the Phonon KCM. Try `kcmshell5 phonon` or (`kcmshell phonon` if you are still using KDE4). The code for the backend selection KCM is at https://cgit.kde.org/plasma-desktop.git ... ection.cpp

I strongly suggest to port to Qt5 and QtMultimedia, though.

The original post was removed as it was a repost of a very old question for the sole purpose of spamming. Sorry you spent your time to answer.