my mum has the same problem. switch on tivo, no sound from tv until changing channel. intermittent too. some days it'll happen, others not. tried changing the hdmi lead - same problem. have rebooted the tivo numerous times. problem only started happening when she had her original tivo box (3yrs old) replaced with a new tivo just before xmas.
have had 1 engineer visit which was useless. didn't even suggest trying a different hdmi lead (which seems to be suggested here frequently)
so any suggestions please? i can provide details of her account if needed.
I was really fed up with the no sound problem and my local retailer suggested turning off my TV and the Tivo box together. Then on again the universal fix for tech problems. Turn on the TV allow it to boot up then the Tivo box this seemed to cure my problem.
Hope this helps give it a try anyway it's easy to do.