How can I convert .nbu backups directly to .mp3 format?
Converting Nokia Phone Backup Files directly to MP3 Audio Files (.nbu to .mp3 conversion) is, per se, not possible. You can, however, extract any MP3 audio file, which is stored in your NBU backup file.
Use a specialized backup extraction tool, such as ABC Amber NBU Converter, which allows users to extract MP3 files stored in NBU backups.
A converter utility for NBU backups