The reason why the video plays in IE7 and IE8 is that for these browsers the RadMediaPlayer uses Flash instead of the HTML5 video tag. This is not true however for IE9 and above which use the video tag.
In your case it seems that the browser prevents the video from playing. You should be able to reproduce the same problem by placing a HTML5 video tag on the page. Please try updating the browser and test whether this resolves the matter. Additionally please try debugging the application(note that for that purpose you should use only the HTML5 tag) as suggested in this
blog post and let us know of the results.
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