1. Install two packages: “pulseaudio-module-raop paprefs” and “paprefs”. The former allows Ubuntu to use the remote audio output protocol (raop); the latter to configure Ubuntu to enable it.
sudo apt-get install pulseaudio-module-raop paprefs
2. Run "paprefs" in Terminal and you'll see the following dialogue box:
Click the “Make Discoverable Apple AirTunes sound devices available locally” checkbox and you’ve enabled roap.
3. To switch audio output to your AirTune speakers, click the “Volume” icon in your tray, then click “Preferences”, as seen here:
Click the “Output” tab and you should see your Apple AirTunes device listed
More info here: http://www.makeuseof.com/tag/apples-airtunes-ubuntulinux/