Playing movies from USB with Pioneer Pro-1140HD

I had this neat idea of previewing the video material I am editing on the Plasma TV using a USB stick instead of burning a DVD. The video plays just fine, but there is no sound. It turns out that even an MP3 has no sound.

This makes me think I have something configured incorrectly. I obtained and installed the latest version of the media SW from Pioneer and I do not have the external speakers installed. I play all sound via the Monitor Out audio terminals to a Sony AV Receiver.

Can USB sound be heard only via the speaker output terminals?

Thanks

TV from antenna input has sound, so the monitor output is alive, theres no sound when playing anything off of the USB stick.

When it didn't work the first time, first thing I did was take the USB stick to a computer and try playing the movie, worked fine. Put an .MP3 on the USB stick and back to the TV, no sound with the .MP3 either.