Thanks for that, Rob.
The nearest I could get was on the BIOS page I was already looking at - AC97 Sound Device which sounds like "onboard sound" to me. According to the motherboard instruction book it should be set either to Enabled or Disabled but on the screen you can only select Auto or Disabled, I suppose that's the same, and it was already on Auto so that option seems to be covered, or as near as I can get.
It might be worth trying to re-install the sound drivers, I suppose, although why they should have disappeared is beyond me (as are most things techie!)
I'll ponder that over the weekend.
GG