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If you have an HDTV, get a Mini Displayport to HDMI adapter. It would plug into the Thunderbolt port on the new Airs, and the MDP port on the old ones. I believe this year's and last year's support audio over HDMI.
 
Mini Displayport to HDMI adapter and you will get the audio as long as the adapter is capable, which is worth noting...

Mine is by Kanex and works as advertised
 
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