Stream iTunes music to many PCs and/or Macs at the same time
Get a copy of Airfoil for the sending machine and you will be able to send one audio stream to many devices. You can buy a single use licenses or get a discount if you buy multiple licenses, but the software is free to try and works for 10 minutes to be sure it does what you need.

There are free receiver software for Linux, Windows, Mac, iOS, Android as well as buit in support for AirPort express, Apple TV, and Boxee.