I just released raop-client which lets you stream music to your Airport Express with ruby.
Just install the gem:
gem install raop-client
Here is a sample program which takes decoded input from stdin:
require 'raop-client'
raop = Net::RAOP::Client.new(ARGV[0])
raop.connect
raop.play $stdin
raop.disconnect
Just use the program like this:
% lame --decode -q some_song.mp3 - | ruby streamer.rb 192.168.1.173
You'll have to supply your own mp3 and Airport Express IP address. Also, raop-client currently doesn't decode music, so you'll have to decode the music yourself.