Airfoil is made for what you want to do. It will send any audio on the computer to any Airplay enabled speaker. I'm with kornchild on this one though, if you have an iOS device with the right apps and a good WiFi network you can't beat Airplay.
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64GB iPad 3, 32GB iPhone 4, 1G 16GB touch, 4G & 6G Silver 16GB nano, 5G 60GB iPod, 1G AppleTV, 2G ATV, 2.5GHz 17" MacBook Pro
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