iTunes 11 has a bunch of small new features that will make you smile, and if you have a couple of speakers in your house that are AirPlay compatible this newly discovered feature will certainly make you happy.
When you connect multiple AirPlay devices to your iTunes 11 Mac you can now control the volume output of those devices through iTunes 11 rather than having to manage each device individually through it’s own UI.
To find the setting just click the AirPlay icon next to the main volume slider in iTunes 11, then choose the “Multiple” tab. You’ll see all of your AirPlay devices and a volume slider for each device. You can adjust the volume individually so that your music plays louder in your kitchen than in your living room.