BR4 format used in iDrive for BMW is based on MP3 format, yet is not playable in standard media players. You have to first convert it to standard MP3 with the bmwconv or similar utility.
Open BR4 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