Turning your entire house into one giant speaker is not only possible, but likely already probable. That is, provided all of the speakers in your house are AirPlay enabled. If they are, then it is surprisingly simple to send the audio from your iTunes music library to all of your speakers simultaneously.
To start, you will need to check and see if all of your speakers are truly Airplay enabled. If not, here are some suggestions for different types of AirPlay-enabled audio setups; or if you prefer, how to make your existing audio equipment, no matter how old it is, AirPlay enabled.
AirPlay audio equipment
Home theater receivers - More and more home theater components are becoming network devices, which includes adding in support for features like Apple’s AirPlay. A while back I took a look at Denon’s AVR–4311CI and its built-in AirPlay abilities. The advantage of using such a setup is that you can play your music library through some of the best audio equipment and speakers available. Sites like Crutchfield make it easy to search for similar AirPlay enabled component audio products from reputable brands like Yamaha, Marantz, and Denon if you are looking to upgrade.
Dedicated speaker systems - For smaller rooms you may not need something as large or powerful as a component-based audio system. Another way to extend the reach of your music library around your home is to purchase a dedicated standalone speaker system that is AirPlay enabled. While many of these solutions cannot plan any music on their own, they will connect to your home network and can play music from any of your iOS and OS X devices. Apple has a list of such speakers that it recommends in its online store that include Bang & Olufsen, Bowers & Wilkins, Libratone and Bose to name a few.
Mobile speaker systems - Not every corner of your life needs to be wired for sound; sometimes, you need to go wireless. For a truly mobile audio product, what you need to look for is a W-iFi enable speaker that supports AirPlay. The iHome iW1 AirPlay wireless audio speaker is just such a device. And with its rechargeable battery freeing you from being next to an outlet, you can certainly move it around to where you need it most. As with the other solutions, it is important that your AirPlay speakers are all on the same home network.
Apple TV - Provided you have an HDMI port, you can use the Apple TV to extend the reach of your music library to your television set. Surprisingly the Apple TV also comes equipped with an optical digital output (sometimes called S/PDIF or TOSLINK) in addition to the HDMI port that you can use to connect the Apple TV directly to your high-end component-based home-theater equipment. This does limit you to some of the more modern and expensive home-theater products. And unfortunately you still need to connect the Apple TV to a display in order to configure and use it.
Airport Express - Not as widely known is the fact that Apple’s Airport Express is more than just a wireless router, it is also an AirPlay enabled device. The advantage it has over the Apple TV is that you can configure the Airport Express remotely without having to attach it to a display. It can also connect to older audio components. With the ability to support both standard audio out as well as optical digital out, the Airport Express can bring your music library to almost any audio component you could possibly have on hand. Just be sure you have the right cable; audio to RCA (separate right and left) or audio to TOSLINK (mini-TOSLINK). You can even use a straight audio to audio connection. About the only limitation is that the speakers will need to have their own amplifier, be it built-in or external.
Full house of sound
Desktop iTunes as the source - While it is true that any app on either iOS or OS X that has support for Apple’ AirPlay technology can send their audio output to any one of the afore mentions speakers, it is only Apple’s own iTunes software on the desktop that can output the same audio to multiple AirPlay enabled speakers at the same time. When playing music in iTunes on the desktop you will notice at the top of the menu — where you select which AirPlay enabled speaker system to use — there are two options to choose from; Single and Multiple. Selecting Multiple will allow you to direct your audio output to more than one AirPlay enabled speaker system at a time. This is what will enable you to play the same song in every room, turning your whole house into one giant speaker. There are even separate volume controls for each AirPlay speaker that allow you to balance the sound around the house without having to be in every room.
Sorry, no Bluetooth - Unfortunately the ability to include additional speaker systems stops short when it comes to Bluetooth. While you can pair Bluetooth speakers with any OS X or iOS device and use the Bluetooth speakers as the default sound output for that device, you cannot add that same paired set of Bluetooth speakers to the list of multiple AirPlay enabled speakers. So if you were thinking you could add your Bose SoundLink or your Jawbone Jambox to your whole house speaker system think again.
Third-party limitations - Another limitation to this setup is that you can only use iTunes on the desktop to send the same audio to multiple speakers simultaneously. You will not be able to do this using any of the AirPlay enabled apps on your iOS device as they can only send their audio output to a single set of AirPlay enabled speakers. This is also true for any AirPlay enabled OS X apps you may have like Vox a popular and easy to use iTunes replacement app on OS X. They too are limited to sending their output to just one AirPlay enabled speaker at a time.
iOS remote app - The good news is that you can control the whole setup directly from your iOS device using Apple’s Remote app. With it you can select which music you want to play, independently control each speakers volume, and even control the playback of the audio. To do so you will need to enable Home Sharing on both your desktop version of iTunes and your iOS device using the same Apple ID.