Airfoil... 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.
Mac Platform: Intel
OS version: 10.7+
Processor type(s) & speed: Intel
RAM minimum: 2048MB
Video RAM: 512MB

more information: http://www.rogueamoeba.com/airfoil/mac/