I had this problem in Chrome because the default video player is QuickTime, and the latest version of the plugin is broken in chrome. The solution is to use another player such as VLC.
In Chrome > settings > show advanced settings > Pravacy > content settings > disable individual plug-ins > disable QuickTime Plug-In 7.7.5 that specifically deals with mpeg, mp4. In my Chrome it's bottom quick-time plug-in.
Then install VLC, and your problem is probably solved.