Okay, just to confirm, is this a Windows 10 beta version, or is it the official release?
I did some further research, here are some things I've discovered. First off, if by chance you're trying to run Big Box over Remote Desktop (probably not likely), it probably won't work. Just had to say it just in case.
Secondly, I'm not exactly sure, but I think Windows Media Player is actually different than the Windows 10 default media player. It's a different app at least. Try typing Windows Media Player (exactly) into the Start menu to load it up specifically, and see if it will play the files. It should be the classic light-blue interface and not the new "Movies & TV" app with a grey background that is the new Windows 10 default. I'm honestly not sure if it makes a difference, but it'd be good to know.