there is a version of Real Producer for Linux
I'm pretty sure it can do what you want.
found this while searching
.I need to be able to convert from RealAudio (.rm) to WAV. I have found tons of WinBLOZE software to do this and I can do it in Linux using vsound and realplay (RealPlayer binary for Linux). It essentially plays the .rm file and stores it as an audio (.au) file then converts that file to a WAV. From there, I would use lame to convert to MP3.