What is RMVB file? 

RealMedia Variable Bitrate (RMVB) is a variable bitrate extension of the RealMedia multimedia container format developed by RealNetworks.

As opposed to the more common RealMedia container, which holds streaming media encoded at a constant bit rate, RMVB is typically used for multimedia content stored locally. Files using this format have the file extension ".rmvb".

What is MP3 File?

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

Extract audio from RMVB to MP3 with 

RM to MP3 Converter or Mini-stream Ripper