I wasn't sure where to put this topic, so I put it here in the general discussion (:
I am not sure if this is a bug or perhaps a feature. My server version is Version 3.0.8500.0.
In short, if a film, per se, has both an MP3 theme and an MP4 backdrop, the MP3 theme does not play when using a client which (perhaps?) doesn't support playing the backdrop when the film is selected. It seems that this is not the ideal behavior, because indeed if Android and web (for instance) don't do MP4 backdrops, they would at least play the MP3 theme if one exists.
I haven't checked the server logs yet - they may or may not have any insight into this behavior.