sorry for the late add to your post but I bought new Panasonic 4KTV two weeks ago and have exactly he same issue. Can see picture but no sound
I've tried changing the HDMI audio output (only options are stero or surrond sound) but that makes no difference, sawpped cables, ports, rebooted, nothing!.Its only the talktalk box that doesnt work....everything elses that connects by HDMI works perfectly so the issue is with Talktalk equipment. It does work on other TVs
Given the number of users with his issue that show on the Web why is there no published solution? This renders the TT box unusable and redundant piece of kit. Calling the helpdesk /service cnetre was a waste of time. Just want to send an engineer to do what that they cant descoibe over the phone?