Shairport is a software that allows you to set up a renderer for airplay streaming. I use it to stream Spotify from Apple devices via a Raspberry pi and a DAC to my living room stereo.
First, install the needed dependencies:
sudo apt-get install build-essential libssl-dev libcrypt-openssl-rsa-perl libao-dev libio-socket-inet6-perl libwww-perl avahi-utils pkg-config
Next, get Shariport and compile it:
git clone https://github.com/albertz/shairport.git; cd shairport/; make
Now you can run it with:
This breaks with iOS 6, see this for a fix.