I gave up trying to export from the film viewer like that, the quality I always got was awful and it still took all day to export. Get a video capture program (easy to find on Google) and capture the picture part of the film viewer while the film is running. It should let you save it in any number of formats. You'll have the option to capture sound as well, or you can edit and add your own soundtrack for the captured file.
The thread Fulmar suggests has a much, much more detailed discussion on how to do this.
