Ok, so most audio editing and converting on the Mac is really easy, but Real's licensing makes converting from .rm a pain-- most of the programs people mention to "convert" the files are really just stream rippers/capturers of one kind or another, which means that a 3 hour show takes 3 hours to convert. Using the method below, you can convert a 3 hour .rm in 5 minutes, depending on the speed of your computer.
The reason it's so complicated to get it done is that you have to take some Real-provided binary files and use them with a Unix program called ffmpeg-- this program actually lies at the core of a lot of software, like VLC.
These instructions tell you how to install ffmpeg, and then use a front end to it called ffmpegX. ffmpegX is a pile of crap, but it's all there is, unless you're comfortable with learning how to use ffmpeg on the command line directly.
http://www.macosxhints.com/article.php?story=20060906153448633Some little points:
If you have occasion to have to rename a bunch of files at once on the Mac (for some reason .rm files hang ffmpegX unless they're renamed to .rm.wav) and don't want to mess with the command line, just go ahead and turn on the Applescript menu (through Applescript Utility, which is in /Applications/Utilities). Then use the Finder script "Replace Text in item Names" to do batch renaming-- search for .rm and replace with .rm.wav. And you can use a program called "Quick Batcher" to batch a bunch of files through ffmpegX.
Luckily, it's only converting Real files that is a pain on the Mac, and WFMU is about the only place I know that still seriously relies on them. flip4mac and something called Perian together make Quicktime (and thus, Safari) able to read any format under the sun-- and of course you can always use VLC. Also, the Real Player on OS X is actually pretty nice. It uses dumb non-standard widgets and so forth, but it's not bogged down with ads and obsessed with getting you to fork out cash for a non-crippled version.