Hi Mike First thing, I played around a bit with the full Finale score for the piece - changed to Garritan instruments, then back to SoftSynth instruments, and, lo and behold, it started playing pro...
Restarting computer didn't help, still no sound. But I changed the settings to play through MIDI instead of VST (still no sound) and then re-set to playing through VST and voila, it started playing...