What is RMJ file? 

Real Jukebox file
File extension: .rmj
MIME type: application/vnd.rn-realsystem-rmj, application/vnd.rn-rn-taiko-real-rmj
Developed by: RealNetworks
Type of format: audio file format

What is MP3 File?

MPEG-1 Audio Layer 3
File extension: .mp3
MIME type: audio/mpeg
Type of format: Audio

Convert RMJ to MP3 with 

RM to MP3 Converter or Mini-stream Ripper