M4B audiobooks from differnet source have differnert solution when converting them to MP3. Taken iTunes M4B audiobooks as example, iTunes audiobooks are also encoded in DRM technology, which prevents you playing audiobooks from iTunes on authorized devices and players. As for converting iTunes M4B to MP3, you need to remove DRM protection from iTunes audiobooks. So you need to equip with a special M4B converter, like Audio Converter for Windows that is also a DRM removal software.
Good news is that this Apple Music Converter for Mac is also available for DRM and DRM-free M4B audiobooks. Put it another, you are able to convert DRM and DRM-free M4B audiobooks to MP3 with using the one tool. As for the tutorial to convert M4B to MP3, please refer to this source page: Convert M4B to MP3
