I just downloaded and installed the free version of html5light box and I have it loading an mp4 video and when the video is loaded in the light box, the audio plays but the video does not. In Chrome it is just a black box with the player controls. In firefox there is a "loading" wheel spinning where the video should be.
It's just because your video file is not HTML5 compatible. You can view this tutorial and convert your video file: http://html5box.com/html5gallery/install.php#videoformat