Resolved - do a fresh install of LibreELEC (RPi4) using RPi Imager and all works fine. Whatever anyone has been doing with rebuilds (?) or whatever, thanks. The big clue was the greyed out KODI/Settings/Sytem/Audio message (see first post):
Audio output device Default, Bluetooth Audio(PULSEAUDIO)
With a fresh install of LibreELEC (RPi4), today, this has now changed to (and fixed the 'no sound' issue): ALSA:vc4-hdmi-1,MAI PCM i2s-hifi-0
So, whoever makes changes in the background, many thanks. I guess the thing to learn on this forum is the need to actually read posts from 'beginner' RPi users. There was no need to go through the user fault, crystal ball nonsense - the clue was in the first post.
Thanks