John, I am having the same problem. I have yet to run into this problem with the constitution course and a few sporadic videos still play normally. I can only assume that is done to further my bafflement. On the videos that wont play, if I click the download video link I am able to watch them on a quick time screen without having to or being able to download them to my computer, while the videos that play normally can be downloaded. Again, must be done for the sake of my bafflement.
Thanks for the info on being able to watch everything on IE, but I assume you, like me prefer not to use IE.
I am a new member so my experience is limited. But I am using Waterfox 18.0.1 (a 64-bit wrapper for Firefox) on Win7x64, and the 1st video of Western Civ Before 1500 playing within the browser okay with no special tweaking or fixes.