I think an amazing feature that should be added to each selected movie (much like TV show), would be movie theme music or soundtrack in the background at 65%-80% volume, (not too over powering).
While doing some digging around the internet, I found an excellent source for movie soundtrack audio streams in SoundCloud playlists, or theOst.com . There are whole movie soundtracks that can be streamed for free, and would make for an excellent metadata agent for streamed theme music.
Having the music in the background builds anticipation for a great movie, and completely brings the theather experience to plex.
Possible Sources I've found:
Sound cloud (under playlist section): Sound Cloud Api
Lastfm (under Album section): http://www.last.fm/music/Alan+Silvestri/The+Avengers