What is RaopX
RaopX is a Apple Airport Express client for Os X based on raop_play.
What is the main difference between RaopX and raop_play
The original raop_play lets you stream audio files to your Airport Express. RaopX streams any audio that comes in on the default input device of your Mac, and combined with the routing functionality of Soundflower this gives you the ability to stream audio from any application you’ve got on your Mac to the Airport Express.
Get and install Soundflower from here.
Then install libsamplerate, e.g. by doing:
brew install libsamplerate