With 10.8, we gained Airplay Mirroring, the ability to wirelessly send everything on a Mac's screen to a television through a second- or third-generation Apple TV. If your computer is a supported model, a new icon will appear in your menu bar when it detects an Apple TV on the same network. Click on that, choose the name of the Apple TV in question, and everything you do on your Mac will show up on your television.
That's pretty awesome. Heck, it's not hyperbole to say it's incredibly awesome. But wouldn't it be better if we could choose to send just the audio instead of both audio and video? We could do that from iTunes already, but what about when you've got your browser open and are listening to Pandora, or if you're using a third-party program like Spotify? Well, slap my…face…and call me Sally, you can do it, and it's easy. All you have to do is go to System Preferences > Sound > Output and control it from there:
Or you can Option-click on the volume control in your menu bar and select the Apple TV as your output device:
After that, whatever you're playing will be piped out to the Apple TV for music-listening goodness, while system sounds (like new email notifications) will not. That's a cool little benefit, methinks.
One caveat: If you've recently used Airplay Mirroring to send both video and audio to the Apple TV, your system may insist on still sending both when you ask it to do audio only. I've had trouble with this when trying to use the Option-click method mentioned above (it's been completely fine for me with the System Preferences way). I've submitted a bug report, people, so I hope Apple's already on the case.