Hi!I fixed same problem on Debian Stretch by installing gstreamer1.0-libav and phonon-backend-gstreamer packages followed by system reboot.
It looks like there is a problem in KDE4/Qt4 version of Phonon to play mp4 video contrainer and only gstreamer with libav plugin (libgstlibav.so) is able to do that for Phonon.
-- Pali Rohár pali.rohar@gmail.com