I have never tried connecting any speaker system directly to the tv itself before, so I may not be of too much help here...but in the main Menu, Setup, then in viera link settings there is an option near the bottom of that list for default speakers, not sure if that will help though.
The only audio out on your set is optical, so that has to be the connection to your sound bar. You then need to go into the set menu and turn off your internal speakers. Look for it in one of the operation menu options, not in the audio menu. That should work.