W4201C plasma tv, cable and dvd player

I have a 42 inch plasma tv (W4201C) ..it works fine when regular cable is the only input hooked up.

When I plug in my surround sound dvd player, the sound on the cable input stops working.  This happeneds even when the surround sound system is turned off.  The input selector is on cable and the picture is unaffected.

Any ideas?

Thanks

Kevin