There is no direct encoder / decoder that will perform this process but there is an alternative way to accomplish it using Winamp:

You can convert the RMX file to WAV using the Tara Audio Video Plug-in. Note: You must have Real Player installed in order for this plugin to work. In order to play Real Jukebox files (RMX), you also need Real Jukebox.

Then you can either usea separate converter to convert the WAV file to MP3 or you can also do this with Winamp using the Chun-Yu Shei MP3 Output Plug-in.

For more information on this process see the RMX to MP3 topic.