under the device manager--->system devices--->System Speaker, has no driver installed... could this be the problem?
Yes, that should be it.

But you installed the driver...briefly had sound...that confirmed the driver being there...and now it's suddenly not there?

You have actually managed to find a weirder problem than I had!

Kind of shots in the dark at this point, but up to trying to reinstall that audio driver again?

As for BIOS, that's even odder...I KNOW it's supposed to be there.