How to convert .br48 files to .mp3 format?
Converting BMW iDrive Multimedia Data Files directly to MP3 Audio Files (.br48 to .mp3 conversion) is possible, even though your options are limited.
It appears that your only option is using the BRx Konverter, which should allow users to export the proprietary BR48 files in MP3 format.
A command line converter for BMW multimedia files
A BRX to audio converter