Thanks. Sadly, I haven’t found anything online to solve the issue. Maybe the problem is that it’s an old tablet and it can’t handle the videos.
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.
Thanks for the tip, I did some research. H264 is the most common format and the one recommended by Unity. It’s the format that consumes the least and hence lags the least. According to Unity, VP8 has indeed better cross-platform support, but it doesn’t have hardware acceleration on old Android devices so they won’t have the power to decode 720p video anyway. On Windows, I think not being able to play H264 videos means something is missing on the computer. On Linux, my games don’t even launch anymore since I updated to Unity 6.2, so I don’t need to be compatible with them I guess.