You put it so nicely and clearly, that I decided to have one last go, and I uninstalled/reinstalled everything as recommended. Sadly, no luck. It would seem there is something fundamentally awry with Dorico with regard to sound, at least on my computer (and it seems I am not the only one.) I am, however, currently getting hold of an external USB audio device, so as to try to avoid the problem (and also latency issues with Sibelius). Perhaps the makers of Dorico might like to consider wrapping its trial version with version 3 of HALion Sonic, rather than version 1.1?
Thanks for your efforts, Traubitz and others.
Philip