It’s still Alpha, but raop_play is an Airport Express client player that will allow you to send audio files, including http streams, to your Airport Express.
Better yet, there’s an ALSA driver included with the package, which will make your Airport Express look just like a second audio output device. Once that’s set up, you can just configure your preferred music applications to send audio straight to the ALSA raop device. How cool is that?
Apple Airport Express Client Player – Link
Stream Music to Airport Express Without iTunes – Link