HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

I updated the mod and tried downloading a different version – the same issue occurred. But through trial and error, I found out why the videos weren't displaying. It turns out that you need to enable Blackout Computer = 1 in the tweaks. That's it, the videos are now showing up. However, this option and a couple of dozen others aren't described here: https://sites.google.com/site/heroes3hd/rus/description/tweaks. I also couldn't find anything about it through a Google search. Maybe there's a document where they are all described?