No sound device in macbook air - Apple MacBook Air

Answer is simple. use headphones plug them in to your computer and pull out slowly. that should activate your computer sound it has happened to me before. i pulled the headphones out too fast and thats when your sound goes away and cant hear speakers.

Apple menu (top left) System Preferences > Sound > Output tab select the Internal Speakers
check your output volume and make sure the Mute checkbox is unchecked.
Plugin a pair of headphones. If you still have no sound. The sound board needs to be replaced, which is attached to the logic board. Meaning the whole logic board needs to be replaced.