Viera Link is Panasonic's branding for HDMI-CEC. I can't access this setting either because I have HDMI-CEC turned off on my receiver (a Denon, it's called HDMI Control). When I turn that on, I can then access the settings for Viera Link, and set it to change the receiver volume instead of the TV volume when I press the volume button. So you need to have a receiver that supports HDMI-CEC, and have it turned on, in order to access Viera Link.
My tv is connect to my Directv box via HDMI. I then have my tv connected to my sound bar via the digital audio out port on the back of the tv. I also have my Panasonic blue ray player connected via HDMI. When I click on the viera link app, all I see is "viera link control" with "player beside it. When I click on this it controls my blue ray player.
After purchasing a pans 50 st 60 I also purchased a sonos play bar to accompany it. Sadly it is impossible to disable the internal speakers on any panasonic tv via viera link UNLESS you have a panasonic HTS.
I am a panasonic retailer and a sonos retailer and have been in contact with both companies direct and there is no work around. Sorry guys. Blame Panasonic