Joy of joys, I fixed it.
I was at a point where I had removed a voodoo audio kext from Extensions, and there was AppleHDA and HDAEnabler present but no output devices and no sound of course.
So I ran the Yosemite multibeast and installed the version of Chimera that was on there (4.something) after seeing somewhere else that the newer versions control the AppleHDA kext or something along those lines.
Hey presto, my audio devices show up again, and are working straight away.
I noted that the AppleHDA kext is still there, but the HDAenabler one is gone now, so Chimera must do away with that I guess.
Please feel free to correct me if I'm talking absolute rubbish about this stuff though as I'm really just guessing with it all and fumbling around in the dark.
Still, it's working, thus, very happy!!!
Except.......
The ethernet is now dead so no internet. Bugger.
Off we go again.
But anyway, the challenge was to get a working Mic plugged in really before this problem second time round. I now have a line in showing as the only input device, and I have a Mic plugged into the blue socket for line in, but nothing appears to come through the mic/input sadly. No idea why. I know a mic into a line in is usually more quiet than in a mic socket, but this is giving nothing at all - no indication it is present.
