I've seen a few people mention this and I suspect (unfortunately) that the fix (workaround) is just to play the video outside of OpenLP but just putting this out there anyway.
I can import MP4 videos into the media manager and they preview just fine - audio and video. However, when I flip them over to live mode, all I get is the sound. I'm using VLC and have tried both the 32 and 64 bit versions. VLC is at the top of the players list within the OpenLP preferences, above webkit and System. The video plays absolutely fine projected when just playing in VLC.
I'm about to go away on a business trip and wll be leaving another church member in charge of operations after ths coming Sunday for 5 weeks and it would make things so much easier for him if I could get this working.