Isn't the echo from the fact that sound travels slower than the light (electrons in the network). The PC's may actually be playing in perfect sync, it is the speed of sound that is giving you the echo.
Look into setting delays on the PC's sound card, but you will have change that as you move around.
Big places use sophisticated systems to compensate for the slower traveling sound to eliminate echoing.
6 TV's in the house on FiOS and we only pay $4.99/month to connect them all!!! Power to the CableCard and WMC7!!!