I have looked at many different ways to stream music from my computer (or my mp3 server, which resides on the internet) throughout the house. There are a couple different types of whole-house audio. One is multi-room-multi-zone audio. These systems are pretty much proprietary but have the most flexibility. The basic idea is that there is one central "server" or more likely "closet" which houses all of your storage and main equipment. This central server is connected (could be wired or wireless) to several "zones" or rooms. Each zone has a controller, amplifier, and speakers. The controllers tell the main unit exactly what to send it to play. Expect one of these bad boys to set you back around $1000 + $200(minimum) per room.
The system I've come up with is much much much cheaper. It sacrafices a little flexibility, but then again, with only two people living in my house, why do I really need a 6-zone system? I have an FM transmitter hooked up to my computer, which broadcasts to normal $20-$100 FM receivers throughout the house. Why is this better? I could never justify spending the $200 or more to put a receiver in my shower, for instance. But a cheap water resistant shower radio? No problem!
This is super easy to do!
My transmitter is the 10mW version, and I am able to go down the street 3-5 houses before the signal cuts out. That is PLENTY of power. Buying the 100mW version is risky, it's definitely against FCC Regulations. Make sure you select a frequency that doesn't overlap with another radio station that one of your neighbors might listen to. That's just mean.