Our van has a VOH642 audiovox dropdown tv with a AVP7000 VCR

I got the video player to play a video by pushing tv/vcr button on remote but now I can't figure out how to hear the sound!!!! The stero in not modified at all in the van, and you can use the remote to make the volume louder but no sound? Can anyone help me??? 5 kids with a long trip coming up soon!!!!
Thank you!