I have a similar problem, videos are just black screen, i thought it's a feature of a demo at first (downloaded game to PC). :D So i've tried and played game in browsers, and PaleMoon (Firefox) does not play videos and a sound, but Chrome plays it. But I noticed some issues with volume controls tho, music bar works ok, while video volume not working when video is playing. Some scenes have no sound, not sure if intended.
Video playing issues happens sometime in other games too, i think their common factor is Unity engine.