.mov and .mp4/.m4a are not the same thing.
Have you made sure your container field is set to display interactive content?
Also, is the container field being stored IN the database, or were you storing it as a reference that is not accessible from your iOS device?
Actually in the Go development guide:
It states on page 10:
FileMaker Go doesn’t support:
1 the Inspector Data Formatting options Images or Interactive content. (Content in container
fields is displayed by the iOS like other iOS audio, video, and PDF files.)
1 setting up container fields to store data externally