Problem solved, although cause was unexpected.

Discovered that the demos that wouldn't play were all mono. Converting them to stereo then allowed them to be heard. It made little sense as to why mono tracks prepared by PG wouldn't play back on BIAB, so had a look at various computer audio settings. Discovered that Sound Blaster 'Z' sound card control panel settings had changed from 'Stereo' to '5.1 Surround' - probably during last windows upgrade - and it was this causing the trouble.

Quite why a 5.1 surround system is seemingly incapable of replaying a mono signal I'm not sure, unless it normally plays via centre-front speaker, which, of course, I don't have.