Is it somehow possible to convert .br25 files to .mp3 format?
Converting BMW iDrive Multimedia Data Files directly to MP3 Audio Files (.br25 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 BR25 files in MP3 format.
A command line converter for BMW multimedia files
A BRX to audio converter