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 4.5.5:
A severe crashing issue caused by a bug in Mac OS X 10.7 Lion has been worked around. This should prevent crashes when using Mac OS X 10.7.
Pulling audio from QuickTime Player and QuickTime-based sources now works correctly on Mac OS X 10.7.
Airfoil Video Player now uses Perian v1.2.3.
Instant On on Mac OS X 10.6 has been updated to version 3.1, to fix an incompatibility with Google Talk. Lion support for Instant On is still pending.
DVD support in Airfoil Video Player is now properly disabled on Lion, pending further updates.
A rare crashing bug with hidden menubars has been corrected.
MuseController is now supported as a remote control source, and provides enhanced metadata as well.
The Radium remote control script has been updated (Requires the forthcoming Radium 2.8.2).