1) Upload the mp4/webm version of the video into the same folder like the mediathek video. The theme will load it automatically if the browser requests another video format. You just need to make sure that the file names of all videos match each other (except the file extension of course).
2) You can also insert following code into the theme functions.php file:
Afterwards Enfold enables you to set 3 different videos (even with different urls) when you want to embed a “video element” into a layout. By default Enfold just allows you to upload/set one video url.