the problema appears on Windows xp using QT 5.3.1. Working with win7 the below code doesn't show anything on winxp.. I recompiled QT with xp support and I have under mediaservice also dsengine.dll but nothing change.
m_layout.addWidget(&m_vw);
m_layout.setContentsMargins(0, 0, 0, 0);
ui.VideoWidget->setLayout(&m_layout);
m_player.setVideoOutput(&m_vw);
ui.VideoWidget->show();
m_playlist.addMedia(QUrl::fromLocalFile(QFileInfo("C:\\file.avi").absoluteFilePath()));
m_playlist.setCurrentIndex(1);
m_playlist.setPlaybackMode(QMediaPlaylist::Loop);
m_player.setPlaylist(&m_playlist);
...then play the video after press a button...
reported to qt support, seems to be a bug also in version 5.4