I eventually tracked down the delay setting on the audio output sub-menu, and ramped it all the way to 250ms, but it doesn't seem to make any difference to the TV screen/speakers. Different settings are stored for the various input sources as well.
The snooker makes a good reference with the clinking of the balls, and there doesn't seem to be any problem today. Maybe it's particular channels and even particular programmes. Any small amount of delay from picture to sound is acceptable, because that's normal in nature. Sound preceding picture can't be tolerated, which is why there are settings to delay the sound to compensate for video processing delays rather than the other way around.