tough issue - can't get sound through video projector

My wife has a Dell Laptop with XP. It runs fine. It has internal speakers but sometimes they aren't loud enough for her audience (she does public speaking) so she plugs in speakers. We plugged the speakers into the laptop and connected the laptop to a video projector. We put a DVD Movie in the laptop and could hear the PREVIEWS of coming attractions just fine. However, when we tried to play the movie itself, the sound went down quite low and we couldn't all hear it. Trying the speakers by themselves without the Video Projector works fine. So it couldn't be the speakers (though I don't know for sure). Plus, being able to hear the Previews worked well so there's something about the actual movie. We tried three movies and it was the same for each one. Anyone care to take this one on?

Hi dconant1

It "might" help to know specifically which brand/model of video projector

And are these movies acquired via commercial purchase/rental.
Or are they home made and burnt to DVD?

The source of the media can be problematic for some display equipment.

Best Regards

The Video Projector is made by NEC #NP40. The disks were purchased/rented from Blockbuster rather than burned. The disks played quite well during the preview section.