Our latest guests have just left our gite and I’ve discovered that the sound is only working in HD programmes on the smart TV. We have a Humax box and a DVD with speakers. The sound works on netflix and youtube etc on the smart TV so I’m guessing it’s something between the humax box and the TV. Can’t see any obvious leads come unplugged.
Grateful any suggestions/advice please.
I don’t have a smart tv (yet), but I would hazard a guess that your gite clients were messing around in the tv sound settings menu.
Thanks Mark for your reply. Doh!!! The humax sound was down at zero. Interesting that the TV still plays HD channels though. Anyway, now all better and I can get on with cleaning. 
You may find that HD plays at very different volume to SD (non HD) - We recently had to fix this on our Humax.
Text found on the web helped us set them to the same >>
Just went into audio/picture settings for the Humax box and had a look. You were right Graham the audio out was set to Multi-channel, so I changed it to stereo - both audio options are now set to stereo.
That’s done the trick !! There is no more big jump in volume when switching between SD & HD channels.
Thanks Peter you are right! We noticed this when we first got our Smart TV. Not noticed it recently - I think maybe one of our smart guests sorted it for us.