Carlo Thanks for further help: System volume was indeed ultra-low, and I have now increased it and produced an audible sound! Thanks, I shall mark you message as the Solution, though at the same time I would warn that System sounds -> Touch sound does not affect this particular operation. Which leads to my next question: can I change this sound? I have been able to import sound-files for various app sounds, but I can't see any way to use an imported sound for the lock/unlock action (note my change of description - I find that the unlock sound comes not when the unlock button is pressed, but when the number-code or other security has successfully unlocked the device).
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