Thanks -- I was thinking the same thing and actually saw some 35 foot RCA cables on monoprice that were really cheap. I wasn't sure if such a weak signal would travel 30 - 50 feet between receivers.
Here's my solution. I thought I'd post this in case anyone else wants to have whole-house audio without running any more wires.
Background: I want whole house audio. I have a receiver in my basement that powers six speakers throughout the basement bar & media room. I have a second receiver in my living room that powers speakers in a living + family room. I have a third receiver in my garage that powers speakers in the garage and patio area. I have a computer in my kitchen with standard speakers.
Goal: Play an audio source from the main kitchen computer that would be able to stream wirelessly throughout the house to all three major audio receivers. The audio source shouldn't be reliant upon one computer program (iTunes).
Problem: Parts of my house do not get adequate WiFi signal.
The Solution: I found a program called Airfoil for $25. Airfoil will hijack any audio stream on your computer, whether it's iTunes, Winamp, Spotify, or a Chrome window playing YouTube. Airfoil will stream this via DLNA to any PC loaded with that software, to an iPod or iPhone device, or an Airplay device like the Airport Express.
Implementation: I needed three devices to receive audio from the main computer. I used my old iPhone 3G and my iPod touch 4th gen. I purchased a used Airport Express that I connected directly via ethernet cable to my main Uverse 2Wire router that sits next to my computer. The living room and basement receivers were too far away from the router to get a consistent streaming signal, so I needed to set up the Airport Express as a wireless access point that would basically extend the range of my network. Airfoil would stream audio to the basement via ethernet to the AE, and the AE would in turn rebroadcast that signal to the living room receiver just one floor above it. The trick was all in the router setup. The AE (or second router) needed to be in bridge mode where DHCP was disabled, it needed a static IP, and it needed to have a different channel than my main router. The SSID and passwords were the same on both routers. The key is that when a device turns to the second router because the signal is stronger, it still gets routed to the first router for all communications. It is important for Airfoil be able to see all the devices on the network to stream to them.
Hope this helps someone else looking to do something similiar. The only wire I used was a pre-existing ethernet cable, and now I have whole house audio for only the cost of the program and one Airport Express.