My Sony Bravia TV has an "Audio Out". This is a pair of RCA pins on the back of the TV. The Audio Out routes sound from currently active input. The settings menu should have settings to mute the TV speakers as well as set if the volume settings (from remote) affect the audio out.
I don't believe there is an easy way to mute TV speakers alone, so every time you connect the headphones you might have to go through the settings menu to have sound come from the headphones alone.