MacBook Air OSX 10.8.5: how to fix AirPlay mirroring after security update, no audio on TV

Hello all,

I updated my MacBook Air (mid 2013, OSX 10.8.5) this morning with the new, urgently recommended, security update. Now when I use AirPlay Mirroring with our Apple TV to watch a show on the plasma TV, we get the video but no audio on the TV. The audio track only comes out of the MacBook's speakers.

I saw others had problems with various features of AirPLay mirroring with Mavericks OS after this security update. Any one have a Mountain Lion problem like this, and how do I fix it? Not a happy camper...

Cinilla