Thanks so much for the reply.
The onboard sound is disabled in bios - its not appearing in device manager so that seems ok.
The creative audigy fx (PCIe) isn't working atm with any drivers as original post - but has, with new clean install, produced some garbled noises (not with every new install - I'm in the 20's now) but as soon as I update to any other driver, I get no sound at all.
I am about to swap out the Creative with the Xonar once again and try over because I just can't remember what I've tried now, but will certainly try the link in your reply.
I mention that the 2 cards use different PCI/PCIe slots to rule out mobo faults (hopefully).
Also, yes I and the (so far 6) techs have done all the obvious troubleshooting methods including safe mode deletes/installs etc.
I have also upgraded my bios to the latest version Asus provides although this is described as windows 8 compatible.
I've just received an email from Microsoft inviting me to a tech session with them tomorrow (this will be the seventh tech guy I have dealt with). I'm assuming an email from them inviting me to a timed (midday) session without requesting it, is unusual and am hoping it's a senior tech that hopefully won't charge me.
As I write this I am exploring the possibilty of 'Fast boot' possibly being the culprit, although this maybe just my fantasy - I read somewhere that it works by taking a snapshot of several different bits of the kernel (including drivers) and uses this snapshot to speed a reboot. This may be total rubbish but I am exploring it because I've just ran out of ideas.
Sorry for the massive reply (I've tried to paragraph it for ease of reading).
And thank you once again for the response - it's so nice to know there is a community out there who like to help