A br5 to mp3 conversion is mostly related to attempts of conversion between BMW iDrive audio and MP3 format. It should be possible to export BR5 audio files MP3 audio using the bmwconv or BRx Konverter programs. It appears that the BR5 format is based on WMA - Windows Media Audio format, albeit with some differences, which makes it incompatible with standard multimedia converters.
Open BR5 file Open MP3 fileA command line converter for BMW multimedia files
A BRX to audio converter
An audio data converter for Mac used to convert audio from BMW car system to MP3, WMA and MP4