Here are all of the known steps to recover your MacAir audio:
1. Reboot – rebooting your computer is a tried and true method for "fixing" all sorts of problems. Sometimes your computer simply gets confused.
2. Repair Permissions – this is a good thing to do regularly anyway, especially before and after you install new system software updates. Repairing permissions via the Disk Utility is something that is easy to do but can take a little bit of time.
3. Reinstall the latest OS X Software Update – this is a great fix for all sorts of issues as well. Go to the Apple site and download the latest Combo Update for your version of OS X. And be sure to Repair Permissions after installing the combo update.
4. Start in Safe Mode – try booting in "Safe Mode" by holding down the Shift key when starting up. Check to see if the audio is working after that. If it does work, there may be some other type of conflict going on and I would continue with the tips below.
5. Delete the BezelServices preferences file – this is a mysteriously magical file. It controls those on-screen images that you see when you change the volume, eject DVDs/CDs, change brightness, etc. But it also seems to be the source of all sorts of odd issues that you can never figure out, and tends to get corrupted. But the fix is easy and relatively safe. Go this preference file (location shown below) and just delete it and reboot. Location & filename:
6. Plug in a USB headset – sometimes your Mac simply gets confused and needs to be literally poked by another audio output device. If you have a USB headset or speaker, plug that in and see if you get audio output through that device. If you do, that’s great! Then try unplugging the USB device and see if your on-board audio output device starts working. (Give it a reboot as well just to be sure it “sticks”).
Let me know which solution works for you!