How to convert .br28 files to .mp4 format.
Converting BMW iDrive Multimedia Data Format directly to MPEG-4 Videos (.br28 to .mp4 conversion) might be possible, even though your options are limited. It relates to users attempting to encode BR28 audio to MP4 video.
It appears that your only option is using a special utility, such as BRx Konverter, which should allow users to import BR28 audio and export it in MP4 format.
A BRX to audio converter