As far as Real Audio goes... it's a Real Pain... I don't know exactly what your server is or what sort of content your are providing, but if you are serving up relatively short audio, you're better off going with Flash 6 and loading MP3s with a preloader progress bar. Flash is more stable, has a larger user base, and you don't need to worry about server issues. If you are doing streaming content, then it's a different story. And MP3 also gives better sound quality. If you need better compression, wait a few years for AAC to become popular...
Hope that helps.