.M4B File Extension
File TypeMPEG-4 Audio Book File
What is an M4B file?
Audiobook file based on the MPEG-4 container format; typically compressed with .AAC encoding; nearly identical to an .M4A file, but denoted as an audiobook and can be "bookmarked" by supporting audio players.
The M4B extension is used by audiobooks downloaded from the Apple iTunes store. These audiobooks may be copy-protected using Apple's FairPlay DRM, which means they can only be played on authorized computers or iPods.
Winamp can be used to play unprotected M4B files on Windows computers. However, the settings for the MPEG-4 plug-in may need to be modified to support files with an ".m4b" extension.