It's taken a while to work out, but.. When my Tivo is in Dolby Digital mode, there's no sound on C-Beebies or CBBC-HD. The SD equivalents are fine - and it resolves when PCM is enabled.
I'm using optical cable-connection from TiVo to a home-cinema amp, but only for stereo rather than surround sound. Not therefore, the end of the world but definitely a bug I think.
This feature was removed in a recent downgrade and Virgin Media no longer support optical output, much to the disgust of many of their paying customer.
Here is just one of many threads on the subject.
Ah yes, I did wonder if it was related - just seemed odd at the time that SD was fine and it only affected HD. Oh well, I'll stick with PCM-stereo for now then.