Thanks very much for the reply.
The "until recently" was that I was out of the country for two weeks and when I returned the sound was gone. My wife and kids had access to the PC in that time
However, I am also having trouble with some of my hard disks and suspect that that may be the problem. I have just now switched the pc off, cleaned and reconnected the IDE cables, rebooted and, what do you know, the sound works...
Sorry for the false alarm.