We do not know about any useful or even possible b1 to mp3 conversion at this time. If you are looking for anything specific, please let us know in the discussion below with as much relevant information as possible. Nonetheless it is also possible that you have some .mp3 music compressed in a .b1 archive and you are trying to get it back. In this case download B1 archiver and decompress the archive to get your music. But that's not a conversion.
Open B1 file Open MP3 file