Android (rooted): AirAudio is a utility that lets your Android phone or tablet play nicely with your AppleTV, Airport Extreme/Express or any other AirPlay compatible device in your home entertainment system. Best of all, it's free.
Your Android phone needs to be rooted for AirAudio to work. You'll be able to push any audio from your Android phone -- whether it's a streaming music or pre-recorded audio to a set of AirPlay compatible speakers or audio devices. Once installed, all you have to do is select the available AirPlay devices on your network, set the output volume, and listen while your music comes out of the big speakers instead of the small ones. You can even play your Android's audio through multiple Airport receivers at the same time.
The app doesn't technically require Ice Cream Sandwich, but ICS users will get the most benefit. There are two audio output modes: "system audio" and "microphone". Microphone is great if you want to speak into your phone and hear it through the speakers, but "system audio" is what you really want to use. On pre-ICS devices, "microphone" is the only option, and it essentially plays your audio on your phone and uses microphone mode to pass that audio to your AirPlay speakers. It's a little echo-y, but it works even on unsupported devices.
AirAudio is free, and it's a great option if you're rooted and running ICS. If you're not, it's still worth a try if you can't find another app that bridges the gap between your Android phone and your AirPlay speakers. Hit the link below to grab it.
AIrAudio (free) [Google Play]