What happens if you plug a pair of headphones into the headphone socket on the side of the TV? Is any sound heard through the headphones?
This might help establish if the TV can produce any audio at all. There seems to be a separate setting for the headphone volume in the sound menu according to the manual.
A bit of Googling throws up a hidden function called 'Hotel Mode' for Panasonic TVs
https://discover.hubpages.com/technology/Hotel-Mode-Panasonic-Televisions
and one of the comments mentions using it with a soundbar to mute the internal speakers by setting 'Maximum VOL level' to 0 (which then allowed the sound only to come from the soundbar and still be controlled by the TV remote).
Do you have any tech wizards in the family who may have enabled this feature to get sound only through the soundbar?
Some other references mention this feature becoming stuck on even after a factory reset.
The hotel mode feature does seem to have the potential to lock you out of your TV so be cautious in how you use it when investigating! You could, however, try calling up the function and see if any of the settings may be causing your sound issue.