Most television systems have an audio output connection. Even though this is not designed for headphones, it is possible to directly hook up your headphones to the connection ports with the help of an adaptor cable. When doing so, the audio from the television's built-in speakers does not turn off, so you are able to hear the audio from two different locations.
Plug the red and white RCA audio ends of the cable into the red and white "Audio Out" ports on the television system.
Insert the cable running from the headphones into the 3.5mm female end of the adaptor cable. This takes the stereo output of the television and sends it to the connected headphones.
Turn on the TV. The audio automatically flows from the TV to the hooked-up headphones.