No sound on MacBook Air
I cannot see 'output speakers' when I click on 'Sound' 'Output' for MacBook air 2011 - when I hit the 'increase volume' key I see a 'speaker icon' and underneath is a circle with a line through it.
Earlier displays & monitors
Check the Audio Midi Setup utility to determine where the output is.
Check that you don't have a headphone jack or USB audio device connected.
Check that Airplay is not on. You can do that by temporarily disabling both bluetooth and Airport in system preferences.