MacBook Air 2011 running Mountain Lion.
When playing video with VLC and air play mirroring to Apple TV, video plays through TV fine but audio continues to play through the MacBooks speakers. I had watched many videos without this problem and then occasionally the sound wouldn't mirror when starting a new mirroring instance or new VLC instance. Restarting the mirroring and VLC several times would often resolve the problem eventually. I couldn't detect why it would sometimes work and sometimes not. Now it won't resolve despite rebooting the computer and reinstalling VLC.