In this podcast, David Woodbridge gives a demonstration of Airfoil for Mac, an application that allows you to play audio on multiple AirPlay devices simultaneously.
Send any audio from your Mac to AirPort Express units, Apple TVs, iPhones and iPod Touchs, and even other Macs and PCs, all in sync! Use Airfoil with web-based audio like Pandora, music services like Spotify, Rdio, MOG, and WiMP, or any other audio playing on your computer. Airfoil for Mac gives you your audio on AirPlay devices all around the house.
Airfoil for Mac can be purchased from the Rogue Amoeba website for $25.