Converting .br3 files to .mp4 format.
Converting BMW iDrive Multimedia Data Format directly to MPEG-4 Videos (.br3 to .mp4 conversion) might be possible, even though your options are limited. It relates to users attempting to encode BR3 audio to MP4 video.
It appears that your only option is using a special utility, such as BRx Konverter or bmwconv, which should allow users to import BR3 audio and export it in MP4 format.
A command line converter for BMW multimedia files
A BRX to audio converter