I was mistaken earlier - sound does NOT work with the DVD function, though it does with the TV tuner.
I tried Ch UP and DOWN together but it made no difference (tried 3 times).
That is correct.
No, Richard. No sound from DVDs, AND no sound from HD!
No. Box was playing a new DVD (in silence) and nothing happened at all when I held the Ch UP and DOWN buttons.
If Ch U/D are depressed while machine is in standby mode nothing happens at all.
If Ch U/D are depressed while the tuner is live (playing a free to air broadcast) playing stops, the tuner re-registers all channels, and when that is complete the option to select power saving mode is presented. That's it.