.M4B File Extension
What is an M4B file?
An M4B file is an audiobook most often used by Apple iTunes and Apple Books. It contains audio that is stored in the MPEG-4 format and compressed using AAC encoding. M4B files are nearly identical to .M4A files except that they support book-related features, such as chapter breaks and bookmarking.
Apple copy-protects the M4B files they provide using FairPlay DRM. This means M4B files that you download via iTunes or Apple Books can be played only on authorized computers and Apple devices, using iTunes or Apple Books. So if you are having trouble playing an M4B file, it might be because you are not playing it in iTunes or Apple Books, or because your device is not authorized to play it.
How do I open an M4B file?
Some free audiobook libraries, such as LibriVox, also provide audiobooks in the M4B format. These files are not copy-protected, so you can play them using third-party media players such as VideoLAN VLC media player.