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