I would think that audio kext(s) might not be loaded properly.
my experience with Lion;
I tested AppleHDA.kext by reinstalling it again and rebooted the OS, and my audio was gone. I had to restarted the OS again and then I got my sound back.
So finally you are right sound is not working anymore today. I don't understand !!!
So except reinstalling what can I do ?
UPDATE : I put 0x75, 0x03 for layout-id and now it works again ... Hope it will be good at next reboot.
UPDATE2 : next reboot no sound
UPDATE3 : I think it depends on how I shutdown the computer, if I choose Reboot it's ok but if I choose shutdown I think there is a power management issue. Maybe due to my gtx 460, I will try with an old card.