In trying to get Safari to open Real Audio content inside the browser, I downloaded the latest Real application, which didn't solve the problem. However, in an act of desperation, I dragged a Real Audio link to the Real application window in the background and it started playing it. This avoided the longer process of clicking on the link, going to the download folder, and double clicking the Real file.
[robg adds: I don't believe Real content will play within the browser. It should, however, play fine in Real Player when clicked in Safari. This hint is a reasonable workaround if, for some reason, clicking Real content doesn't launch its player.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20041022190649471