IE10/11 not playing mp3 plugin

[Sheridan]

We have an issue at the minute with some sites not playing the mp4 videos (with a video player embedded in the page) The player seems to try to play, but ultimately doesn't play the video.

 

It only seems to affect IE10/11 though, older (IE9) browsers and firefox work ok (although we haven't deployed firefox across the network) so I'm a bit baffled. Even adding the site to a safe sites list has no effect. I've checked our group policies etc but it also affect admins so it doesn't look like a permission issue.

[mulderite]

This may be an obvious question, but have you tried compatibility view?