What is RA file? 

RA is the file extension for the RealMedia streaming audio container format developed by RealNetworks.

RealAudio
File extension: .ra
MIME type: audio/vnd.rn-realaudio, audio/x-pn-realaudio, audio/x-realaudio, audio/x-pm-realaudio-plugin, video/x-pn-realvideo
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 RA to MP3 with 

RM to MP3 Converter or Mini-stream Ripper