RealAudio format is still fully closed source, so there are no NDA-free projects supporting it. Although a reverse engineered codec is included in MPlayer?, it is based on floating point arithmetic and is not suitable for the FPU-less XScale? processor mounted on NSLU2.
The Helix project made available to the general public a fixed point codec and the Real streaming format handler, but only in binary form (a static library). The simplest command line player with audio stream support can be built in the following way.
NB: the binary library is available only in little-endian format, so your distribution must be little -endian too (e.g. debianslug).
Don't worry if the process seems to hang at certain points: some libraries (like the RALF lossless format) are not available for our SYSTEM_ID, but one can live without them.
You should end up with a 'release' directory with the 'splay' application in it, along with the codecs as dynamic .so libraries. Move all of them to your box. You should be able to listen to the BBC now (http://www.bbc.co.uk/radio1/realaudio/media/r1live.rpm(approve sites))!