This seems similar to the resolved bug https://bugzilla.wikimedia.org/show_bug.cgi?id=56405 ("?embedplayer=yes broken for videos with width less than < $wgMinimumVideoPlayerSize (800px)").
Examples:
https://blog.wikimedia.org/2013/11/08/open-letter-free-access-wikipedia-south-africa/ - video is blank
https://blog.wikimedia.org/2013/10/21/scientific-multimedia-files-get-a-second-life-on-wikipedia/ (one of the cases from bug 56405) - all three videos are blank, as is the audio recording at the bottom
https://commons.wikimedia.org/wiki/File%3ACienciaaberta_gt2013_abertura.webm?embedplayer=yes (one of the cases from bug 56405) - video is blank, JavaScript console in Chromium shows the following:
Uncaught TypeError: undefined is not a function File%3ACienciaaberta_gt2013_abertura.webm?embedplayer=yes:45
(anonymous function) File%3ACienciaaberta_gt2013_abertura.webm?embedplayer=yes:45
Uncaught TypeError: Cannot read property 'profile' of undefined load.php?debug=false&lang=en&modules=startup&only=scripts&skin=vector&*:2
(anonymous function) load.php?debug=false&lang=en&modules=startup&only=scripts&skin=vector&*:2
jQuery.extend.each load.php?debug=false&lang=en&modules=jquery%2Cmediawiki%2CSpinner%7Cjquery.triggerQueueCallback%2Cl…:5
jQuery.fn.jQuery.each load.php?debug=false&lang=en&modules=jquery%2Cmediawiki%2CSpinner%7Cjquery.triggerQueueCallback%2Cl…:2
$.fn.embedPlayer load.php?debug=false&lang=en&modules=startup&only=scripts&skin=vector&*:2
(anonymous function) File%3ACienciaaberta_gt2013_abertura.webm?embedplayer=yes:62
Version: unspecified
Severity: normal