How to Stream Music with AirPlay
If you’re using an AirPort Express portable wireless Base Station or an Apple TV, you can ship your songs right to your AirPort Express or Apple TV from iTunes, and from there to your home stereo or boombox! (Or pick up an AirPlay-enabled sound system, and you can eliminate the stereo or boombox and the additional Apple hardware entirely.)
After your AirPort Express Base Station is plugged in and you connect your home stereo (or a boombox or a pair of powered stereo speakers) to the stereo minijack on the Base Station, you see a Speakers pop-up list button appear at the bottom of the iTunes window. (If the Speakers button doesn’t appear, choose iTunes→Preferences to open the Preferences dialog and then click the Devices button on the toolbar. Make sure that the Allow iTunes Audio Control from Remote Speakers check box is enabled.)
Click the Speakers button, and you can choose to broadcast the music you’re playing in iTunes across your wireless network. Ain’t technology truly grand?