You can plug your stereo system or speakers into the audio port on your AirPort Express using an audio cable, or you can connect your AirPlay-enabled output devices to the same local network as your iTunes library.
If you connect your computer to the same network as your AirPlay-enabled device, you'll see an AirPlay button when you open iTunes 10 or later on your Mac or PC. To control where you stream your content, click the AirPlay button.
Add AirPlay to your current network
Connect your stereo system to the audio port on your AirPort Express using an audio cable and plug it into an electrical outlet. After you connect, AirPlay wirelessly links to your existing network, letting you play music in any room with AirPlay-enabled output devices.
AirPlay-enabled output devices other than AirPort Express may connect to the network in different ways. See the instructions from the manufacturer for more details.
Listen through multiple speakers
With AirPlay and iTunes 10 or later, you can listen to music through your computer speakers and multiple AirPlay outputs.
The maximum number of outputs is three to six. The number that works for you depends on your environment (such as building composition and local radio interference), distance to the outputs, and available network capacity relative to your other usage.
Control multiple speakers
You can use the Multiple Speakers window to control the volume of your output devices.
The sound from your AirPlay output device may seem louder or softer because of the acoustics in the area, and the sound may be different compared to the sound from the same device in a different area.
You can adjust Master Volume in the Multiple Speakers window or use the main volume control in the upper-left corner of the iTunes window. If you set the softest output to match the Master Volume, you have more precise control over that output when you adjust the Master Volume. You can set loud AirPlay outputs to a lower volume to better control the extra power.
Control playback on the go
You can use the Remote app to control iTunes or Apple TV from your iOS device. If you're streaming from your iTunes library and step away from the computer, take your iPad, iPhone, or iPod touch with you to control your iTunes library. You can download Remote, a free download from the App Store.