Darren, to clarify:

I did some test recordings on the weekend, setting the TiVo up to record two consecutive programs on the same channel, then switch to the next channel and record a pair of consecutive programs, etc ad nauseum.

The problem has occurred on ABC, Channel 7 and Channel 9. I mostly notice it on ABC, as I have set up a whole host of Season Passes of kids shows for my son - and many of them are on, back to back, on the ABC in the mornings. As mentioned before, I now have excellent TV reception, a (possibly over-specced) new antenna, quad-shielded cabling and F connectors throughout the entire house. I have LOS every one of the Mt Coot-tha transmitters, the furthest of which is ~5km away.

...this only happens for recordings made through the PAL tuner?
Yes - the only source I have is PAL tuner. I don't have a set-top box or anything else.

So does the sound only disappear when a secondary recording is made on the same channel...
Yes - if (to execute the To Do List) the TiVo has to change channel, then the first recording on that channel is always fine. Second and subsequent recordings on that channel (mostly) have no sound. Once or twice the problem hasn't happened as expected, but mostly (95%) the sound goes.

This problem does not <snip> happen randomly while watching live TV?
Correct - if just watching live TV, and nothing is being recorded, then there is no problem whatsoever.
If, however, I am watching TV, and the TiVo goes to record something automagically - then the potential for a problem occurs. If I'm watching Channel X, and TiVo goes to record a show on Channel X, I lose sound - I've seen it happen. If I was watching Channel X, and TiVo goes to record on Channel Y, then there is no problem for that first recording, however see my previous point - again, I've observed this behaviour.

This problem does not happen part way through a recording ...
I think the answer to this is yes - if a recording starts OK (ie: with sound) then it is OK for the whole recording. If a program starts with no sound, then it's stuffed.

Cheers!