Nicecast will take audio from any app and create an mp3 stream. You can then play the mp3 stream in iTunes and stream it to Airport Express.
You will probably need a fairly quick mac to do this. A quick test playing an mp3 in VLC and streaming to iTunes at 96kbps kept rebuffering every couple of minutes (G3 800 iBook).
Edit: Nicecast isn't freeware ($40 but you can try it for free) but I've had great experience with it. If you're looking for something free, go to VersionTracker and search for similar apps ("internet radio" or "audio stream" or something).