page 31 of the TV´s manual:
Allows you to select what controls the audio output. Fixed: Sound via speaker is adjusted with VOL+/- on the remote control unit or VOL+/- on the TV. Sound via the AUDIO OUT terminal is not adjustable and outputs at a consistent level. Variable: Sound via the AUDIO OUT terminal is adjusted with VOL+/-on the remote control unit or VOL+/- on the TV. Sound via speaker is muted"
Not sure if it would work similar with the digital audio out port, but it´s possible to use both TV´s speakers and headphone at the same time by the audio out port.