This issue has been a pain since I purchased my ZT several weeks ago. My Vizio sound bar has a Optical connection. Every time a turn on the TV, I have to wake of the sound bar which requires to increase/decrease of the volume on the remote. When you adjust the volume on the remote, it also does the same for the internal speakers. After a period of time, the internal speaker can creep up and mix with the the sound bar. Adjusting the volume in the Hotel Menu fixed this problem and is a great solution for my needs.
If anyone is looking for a solution to turn off/disable the internal speakers, here's a solution: Activate Hotel Mode: 1) Hold down Volume Down button on the TV (right side). 2) At the same time, press the Input button on the remote three times. This should put you in Hotel Mode. If not, try again.
Once Hotel Mode is activated complete the following steps: 1) Select the Hotel Mode tab and turn on. 2) Select the Volume tab and reduce to zero. 3) Select the Volume Maximum tab and reduce to zero. 4) Exit Hotel Mode by turning off the TV via the remote control. Don't press the return key to exit as the setting won't take.
When you turn the TV on you should be good. Works for my ZT but should also work for the ST and VT. Good solution for those using a sound bar with Toslink. Would've been nice if Panasonic had included this feature via the Menu. At least we have a solution.
Bron:
http://www.avsforum.com/forum/167-plasma-flat-panel-displays/1471264-there-no-way-disable-internal-speakers-60-series-pannys-panasonic-tech-support-2.html