Thanks for all the suggestions. Sadly I've now tried them all...and a few others probably best described as clutching at straws...all to no avail. The thing still just sits there in silence.
To answer a couple of questions arising from my original query. The BIOS setting is for the output for front panel sockets. Doesn't make any difference whether it's HD or AC'97 - nothing comes out.
The various drivers appeared to set up different outputs - at least made different devices appear in the mixer. Even those which showed speakers (as opposed to SPDIF) indicated that the speakers were not plugged in. Strangely, using the Realtek driver the Audio Manager showed the rear sockets 'greyed out' - but even though the front panel sockets seemed live no sound came out.
I think I've spent enough time on the darned thing now! It's too much trouble to send the thing back - and I can't help feeling it's more likely to be something I'm doing than a fault on the board - so I think I'll try some cheapo soundcard or USB adaptor.
But thanks again for the ideas.