It should be quite possible to export BR4 audio files used by iDrive from BMW to MP3 audio using the bmwconv utility. Since the BR4 format is based on MP3 format with some differences, that is incompatible standard multimedia converters.
Convert BMW iDrive multimedia data to Compressed audio and music file, mp3 songs or ringtones.
We have found 3 software records in our database eligible for .br4 to .mp3 file format conversion.
Microsoft Windows software - convert br4 to mp3 on Windows
A command line converter for BMW multimedia files
Convert from br4 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 br4 to mp3. 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 br4 to mp3 on OS X
An audio data converter for Mac used to convert audio from BMW car system to MP3, WMA and MP4