My knowledge of AV stuff is almost nonexistent, so I'm probably the last guy who should respond to this.
I watched a video about our VC 19ERD where a guy talked about this. I know it is a different model, but maybe the same applies. Anyway, the guy says that the built in speakers will only work when you are playing a DVD from the "receiver". If you are using the coaxial connection from the antenna or the campground (or your laptop etc), then only the TV speakers will work.
My feelings will definitely not be hurt if someone circles back and corrects any inaccurate info in my post.