Have the same setup. AppleTV in my theatre room and Airport Express upstairs. Can play music from my iPad/iPhone at one or both of those locations. Also can have it play on my iMac in my office.I use an Apple "Airport Express" as my WiFi router, which has a stereo-output jack so you can link it to any stereo receiver or powered speakers with an input. I can stream itunes to it, and there are a few other apps that support it, too (I use a 3rd-party Pandora app a lot). I think Apple TV allows you to do the same thing. It's nice because it gives you a lot more flexibility with speakers and receivers.
Have the same setup. AppleTV in my theatre room and Airport Express upstairs. Can play music from my iPad/iPhone at one or both of those locations. Also can have it play on my iMac in my office.