Do you meet the following Requirements for Airplay Mirroring?:
Requirements for AirPlay Mirroring in OS X Mountain Lion
An Apple TV
- Apple TV (2nd or 3rd generation) with software update v5.0 or later. See How to update Apple TV software.
Supported Macs: AirPlay Mirroring in OS X Mountain Lion takes advantage of the hardware video encoding capabilities of 2nd and 3rd Generation Intel Core processors. Hardware video encoding allows AirPlay Mirroring to efficiently deliver high frame rates while maintaining optimal system performance. The following Mac models have processors that support AirPlay Mirroring:
- iMac (Mid 2011 or newer)
- Mac mini (Mid 2011 or newer)
- MacBook Air (Mid 2011 or newer)
- MacBook Pro (Early 2011 or newer)
It is frustrating that going into system preferences is required when it should automatically switch the audio to the Apple TV without this additional step -- seems to be a bug in Airplay mirroring functionality.
I have found that a reboot gets it working correctly as well. After a fresh reboot, the audio automatically switches to Apple TV when enabling Airplay mirroring. Of course, this extra step shouldn't be needed (just like it shouldn't require going into system preferences).
Slightly easier than opening system preferences/Sound is to hold the "option" key whilst clicking the volume control on the menu bar. This will bring up a menu to select the output devices. (If you don't have the volume control on the menu bar you can enable it in system preferences/Sound.)