Wirelessly Stream Music Through the House

If you're looking to stream audio wirelessly through your house, there are a few different ways to do it.

DLNA
DLNA, which stands for Digital Living Network Alliance, is a wireless standard that many devices support for sharing digital media. There are many DLNA certified devices, including A/V receivers, smartphones, game consoles, and TVs. If you have these devices on the same network, the devices should be able to see one another. If not, programs like Plex can make use of it.

Sonos
Sonos makes wireless speakers that can be controlled from any device through an app. The smallest Play:1 speaker costs $199, the Play:3 is $299 and the largest version, the Play:5, costs $399. These speakers also can work in concert with one another, so if you buy two, you can use them as a stereo pair. These speakers work very well, and are a great solution for whole home audio.

Apple TV
If you happen to live in Apple's universe, it's relatively simple to do this with an Apple TV for $69. You can connect the Apple TV to your home theater system, and use your Mac or iOS device to AirPlay audio and video to the Apple TV.

AirFoil by Rogue Amoeba
This program will send audio from your PC to AirPort Express units, Apple TVs, iPhones and iPods Touch, and even other PCs and Macs. It costs $29 for the Windows version, or $40 for the Mac/Windows bundle.