FIXED! Now this is just bizarre: the problem was still there this morning, no sound on HD channels apart from ITV HD. So I was looking through the sound settings on the Tivo V6 and found this one: Audio Settings --> Dolby Digital It has these options 1 - Dolby digital (it was set to this) 2 - Dolby Digital to PCM
I changed it to (2) - hey presto the sound is back!
So all is now working as it should, but why? I have NEVER changed that setting before, and I live alone so nobody else has changed it either. Unless my cat somehow switched everything on and changed it when I was out ...
Everything worked fine yesterday morning when I was briefly watching a bit of the news around 08:30, I switched the Tivo V6 and my TV (LG 43UJ670V) off, then when I switched them on at 1pm the HD channels had no volume. I use the internal speakers on my TV, and there are no similar settings on the TV for that, besides - I haven't changed any settings on my TV, they are all at the defaults.
Just wondering if there was any Tivo update done during that off time which changed the default setting perhaps? Otherwise - why would it work fine for 11 months and then suddenly change one morning?