I’ve now tried everything suggested so far, and still no luck, sadly.
I did try first removing the windows 10 drivers and then substituting windows 7 drivers from Lenovo, but no difference. I’ve now replaced them again with the windows 10 drivers. I also tried changing the IDT High Definition Audio CODEC via Device Manager. However, it does not have this CODEC installed and already has the High Definition Audio Device. Changing it made no difference.
Playing around with the keyboard in the HALion Sonic editor window has occasionally produced the odd short squeak (of the correct pitch), especially immediately after reinstalling ASIO4ALL just now. But recently I have not heard this sound again. Lights flash and the level indicators show sound is being produced (whether using ASIO4ALL or the ASIO driver provided by Dorico), but nothing can be heard. I have also tried playing around with the small adjustments in ASIO4ALL suggested (e.g. “Always resample 44.1 kHz <-> 48 kHz” / “Force WDM Driver to 16-Bit”) to no avail. I’ve continued to adjust the sample frequency of the computer to match the driver.
However, sound CAN still be heard from other applications once Dorico has been shut down, so there is nothing wrong with my amplifier and speakers. And as mentioned before, Sibelius 8.6 continues to work fine (with its rather limited range of choral and orchestral sounds - hence my interest in Dorico!)
Any other thoughts? Does my DxDiag file reveal anything?