If you plug a 3.5mm jack plug into the socket does the sound still come out of the speakers?
It used to be the thing that plugging in to the headphone jack would mute the speakers. That's a throwback to the days when TVs weren't as menu-driven.
What's more common you'll often find now is that the TV has menu options for how the headphone socket works, and this means it no longer mutes the speakers. There'll be a sub menu to set the volume level independently from what the speaker volume is set as. The catch is that this menu is often a bit buried, so it's not like you can grab the remote and tweak the volume level in the same quick was as you would the main volume.
Be prepared for things to be less that straightforward though.
In a simple world, the easy answer is to use the Optical out on the TV in to a headphone transmitter that either accepts Optical in, or via an Optical to stereo audio convertor. The Optical out would stay live and at a fixed level regardless of what happens with the TV speaker volume. The headphones would have their own volume control.
The reality is that all the additional menu options for TV speakers, headphone settings, HDMI control and possibly some other bits trying to get their oar in means that things don't always work in the way we'd think logical. For example, my TV is a Panasonic, and I can have it hooked up to my surround system via either HMDI ARC or Optical. I'm using Optical because I like to choose sometimes just to use the TV speakers, so I need a 'dumb' signal connection. However, I still want HDMI control for some automated switching such as bringing the TV out of standby when the Sky box powers on, and for changing input selections when the AV receiver is in standby. There are times when the TV thinks it has to mute the Optical output. Not everyone is using a Panasonic TV, or using it in the way I'm using mine, but the point remains that getting these things to work consistently might involve some workarounds.