I hear ya, sk123.
Many installers simply don't bother connecting the TV audio. After all, it's another two or three dollars.
I, like yourself, don't agree with that. I don't always want to use the surround system, particularly when the grand-kids are watching their Dvds.
What input on the TV does the DVD player plug into? The reason I ask this is that usually each video input on a TV has a corresponding audio input, usually a red and white jack.
If those red/white jacks are not yet utilized, that's why your TV doesn't have sound when playing a DVD. That's a simple and inexpensive fix.
Look on the back of the DVD player for a pair of red/white jacks marked "audio out" or some such. Simply run an audio interconnect from the output on the DVD player to the input on the TV. You don't need to spend a fortune here. Go to Radio Shack, Home Depot, Lowes, whatever it takes.
You'll have to turn the TV all the way down when using the surround system but that's about the only way to accomplish what you want. It ain't no big thang, though.