Since 10.3 MacOS X has been able to do this out of the box with the "AU Lab" Application included with the developers tools to configure MacOS X to Send and Receive raw audio or even encrypt the audio during transport using a passkey.
But if you need cross-platform network audio transport this maybe what your looking for.
Wormhole
Audio plugin (AU/VST), send audio over TCP/IP
Version: 2.0.4