Sound not working on MacBook Air mute box is checked and greyed out
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