I saw the recent Nicecast hint for doing audio to multiple audio apps today. Much more interesting to more people (I assume), is this article. According to the article, it looks like Nicecast can be used in conjunction with the iTunes, to send audio from other apps to the AirPort Express. So, if you want to send audio from, say, RealPlayer, this will do that. It takes the audio from the original app, makes an MP3 stream which iTunes can tune in to. iTunes can then be set to send this stream out to the AirPort Express.
Check out the article, it gives a lot more detail on the required setup steps...
Mac OS X Hints
http://hints.macworld.com/article.php?story=2004081800275978