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Hello , m name is Jack
Hi , I`m Jack Perhaps I can help you with this problem
Was looking at all the options for this tv and could not find a headphone jack that would`ve cut off the tv` speakers and could not find a way to set the internal speakers to off through the menu either so the only real way to disable them if you are not using them anyway would be to remove the back cover and either pull their plugs at the main board or simply cut the speakers wires.
If you need help with that let me know.
Here is a picture of your main board and I pointed to the only 2 possible connectors for the tv speakers , yo can simply follow the wires or pull it out while the tv is working to find out which one .
Click the link below and if that link opens a login window popup then just close it with the upper corner X or the "no thanks . continue to view" ;
https://www.dropbox.com/s/lbzogb91fyrwnj2/2015-12-31_0856.png?dl=0The picture is behind that .
Other than that the poor or distorted sound would be caused by the main board itself not the speakers and that board some some $200 to buy , nearly the cost of a new very decent surround system and you would have to remove the back cover to replace it anyway so just as well to simply pull out that plug and keep using your soundbar .
Let me know if you have questions on that .
Happy new year and please do not forget to rate my answer so I get credited for my time with you .