Internal speakers won't turn off when external audio is connected
Normally , you would expect the TV sound to stop when headphones are plugged in. Obviously not in this case. If you have a set of headphones, try them and see if the TV still has speakers on. If it does, then there may be a setting on the setup of the TV to get headphones to work. If the TV speakers stop, then I would think that the Logitech are not compatible with the TV. The only other thing would be is to get into the TV and disconnect the TV,s own speakers. If the wires are soldered on and not terminal connected, carefully cut 1 of the wires off near the speaker. If 1 wire is cut, the speaker wont work. Make sure both the TV and the additional speakers (If they have their own power supply) you are using are unplugged from the power before you go doing this...!