Airfoil allows you to send any audio to AirPort Express units, Apple TVs, and even other Macs and PCs, all in sync! It s your audio - everywhere. With Airfoil you can take audio from any application and send to your AirPort Express units, as well as Apple TVs, and even other Macs and PCs running Airfoil Speakers! Transmit audio from RealPlayer, Windows Media Player, QuickTime Player, and other media players. Send audio from web-based applications like Pandora, Last.fm, and other to the AirPort Express. You can even stream audio from audio devices like RadioSHARK, XM and Sirius radios, around your house.

Requirements
Intel, Mac OS X 10.6 or later.

Version 4.5.7:
Instant On 4.1 now supports Mac OS X 10.7 (Lion), allowing for capture of all System Audio, as well as grabbing audio from already-running applications.
A bug where AppleScripts using the System Audio device would fail has been corrected.
Several assorted crashes and memory leaks have been fixed.