It appears that Chromium, the browser used by Offline Surveys, is unable to support your MP4. You may be able to resolve your problem by converting your MP4 into an OGV, then including both files in your survey.
Here is one tool for converting video files: https://convertio.co/mp4-ogv/
Once you've added the converted file into your graphics folder, you can modify your HTML to look like this:
<video id="myVideo" width="640" height="320" controls>
<source src="[%GraphicsPath()%]Similac TC-HD-1.mp4" type="video/mp4">
<source src="[%GraphicsPath()%]Similac TC-HD-1.ogv" type="video/ogg">
Sorry, this browser cannot display the embedded video file.