I have two Panasonic TVs and on both of them you can, unusually, use both headphones and normal audio at the same time, i.e. connecting headphones does not mute the internal speakers.
If you have a set of earphones try it.
If you have this system then headphone volume is normally set independently, but only via the sound menu, not via the remote.
Your best bet is to get a set of headphones with an in-line volume control (Argos etc)
The alternative is to connect the tv's audio output, normally red/white plugs on the rear of the set, to a headphone amplifier.
Hope this is not too confusing; come back to me if you need clarification.