Having problems? The HTML5 audio/video tags are still a bit buggy.
Try changing browsers; I keep Chrome, Firefox and IE handy, one of them should work.
Or hit the refresh button (F5), that works, especially in IE and Firefox.
When playing videos allow a few seconds for a "pre-buff", that seems to help.
Over the years I've accumulated a number of videos, from flight simulators to racing simulators to trailers for my now defunct YouTube channel (Hacked by the Russians, deleted the lot then HTML5 happened). So all these videos, what to do? Slice'em, dice'em, reformat and load'em here, for our (your and my) enjoyment.