MP4 File Added to Service but Won't Play

As a new user, I'm still struggling to know how to use presenter. I had previously loaded four DVD MP4 files into my service order and they played just fine with a single click on the item. After I attempted to load some MP3 split-track audio files, I must have messed up some settings. The MP4 files won't play now. The "play-pause-back" bar comes up, but is not active. Any suggestions?

I just answered my own question. I had inadvertently set my output to the stage display and the DVD MP4 files will only play when the output is set for the main audience output. I'm sorry if I troubled any of you with this question.