Sometimes, sound output in a MacBook laptop is gone. To restart the sound driver, kill the coreaudiod service - it will restart automatically and sound output will be restored:
John-MacBook-Pro:~ eyal$ ps -ax|grep corea
81524 ?? 0:00.08 /System/Library/Frameworks/LocalAuthentication.framework/Support/coreauthd
82158 ?? 0:00.35 /usr/sbin/coreaudiod
82163 ttys001 0:00.00 grep corea
John-MacBook-Pro:~ eyal$ sudo pkill coreaudiod
John-MacBook-Pro:~ eyal$ ps -ax|grep corea
81524 ?? 0:00.08 /System/Library/Frameworks/LocalAuthentication.framework/Support/coreauthd
82167 ?? 0:00.30 /usr/sbin/coreaudiod
82172 ttys001 0:00.00 grep corea
