To my knowledge there is no way to have it automatically change the way you mention. Your best bet may be to buy a VGA cable, a male to dual female 3.5mm audio splitter, and a standard 3.5 to 3.5 male to male stereo audio cord. Plug the splitter into the headphone jack on your PC and run the stereo cable to the TV along with the VGA cable. That would leave you with an open port for your headphones on the splitter, and you could simply mute the TV or unplug the audio cord running to it from the splitter when you only want sound coming from the headphones. VGA cables support higher resolutions than 1920x1080. So this would still give you full HD display.