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".

WAV File Extension

Waveform
File extension: .wav
MIME type: audio/wav
audio/wave
audio/x-wav
Type code: WAVE
Uniform Type Identifier: com.microsoft.waveform-​audio
Developed by: Microsoft & IBM
Type of format: audio file
Extended from: RIFF

Extract audio from RMVB to WAV with Mini-stream Ripper