There do exist some utilities (mainly German) that can be used to convert special BWM iDrive multimedia files (*.br5, *.br4, *.br3 etc.) to other more common multimedia files like the MP3, MP4 or WMA:
Convert BMW iDrive multimedia data to MPEG-4 video file format.
We have found 3 software records in our database eligible for .br5 to .mp4 file format conversion.
Microsoft Windows software - convert br5 to mp4 on Windows
A command line converter for BMW multimedia files
Convert from br5 file format
The tables with software actions are a good pointer for what a certain program does with various file types and often may give users a good hint how to perform certain file conversion, for example the above-mentioned br5 to mp4. However, it is not perfect and sometimes can show results which are not really usable because of how certain programs work with files and the possible conversion is thus actually not possible at all.
A BRX to audio converter
Apple macOS / Mac OS X software - convert br5 to mp4 on OS X
An audio data converter for Mac used to convert audio from BMW car system to MP3, WMA and MP4