MacBook Air on Yosemite no sound device I just upgraded my 2010 MacBook Air to Yosemite. 
It froze during installation, I shut it down and when I restarted it booted normally to Yosemite.

Now everything seems to be working ok, but I have no sound. My case is not like most cases where it defaults to an external device. I have nothing attached, no monitors, no external speakers.

When I check system preferences/sound, I get no output devices found. 

Here's what I've tried:
- rebooting
- reset NVRAM
- reinstalling yosemite (same freezing problem - waited for 4 hours)
- remove plist files from Library/preferences/audio and restarting

Any suggestions?

Thanks
Reset the SMC:

1) Power down the system
2) Unplug all cables except Magsafe
3) Hold down left Shift + Control + Option + Power button for ~5 seconds
4) Boot machine up

This usually solves issues when certain hardware devices on the device itself aren't responding, especially when multiple do not work.