Sound not working on MacBook Air mute box is checked and greyed out My audio on my MacBook Air is not working. When I go into System Preferences and go to Sound, the mute box is checked and greyed out. My Mac can't play anything including iTunes and YouTube, even system sounds such as the sound when you send an email.

I have reset the NVRAM and repaired the disk using Disk Utility. I also went into Terminal and ran sudo killall coreaudiod but that didn't work either.

When I turn on the laptop it doesn't make a startup chime like it normally would. My headphone jack does not have a red light in it and it shows I have no audio output or input.
Devon, I did some investigating on the apple support site and found this. 

1) Delete both files in YourHardDrive->Library->Preferences->Audio -> (both .plist files in this folder)
com.apple.audio.DeviceSettings.plist 
com.apple.audio.SystemSettings.plist 

2) Make sure to empty your trash. 

3) Reboot 

Let us know if this works! 

Apple Support Reference: https://discussions.apple.com/message/23538716#23538716