If you are using you pen drive for movies maybe you can run them through your computer and have the audio go to a amp or receiver and send the video from your video card into your projector.
As for me, I will be watching movies, videos, video games either from my Playstation 3 or computer, or blueray player, and I have a 5.1 receiver that I send the audio to, feels like your in a movie theater.
Do you have a Playstation 3 ? or Xbox ? perhaps those can play the MP4 files and send the audio to a amp or receiver and send the video out to the projector.