What is inside an M4B
An M4B uses the MPEG-4 container, the same family as .m4a and .mp4 files, and the audio inside is usually AAC. What makes it a book is the extra information stored with the audio.
- Chapter markers with titles, so listeners can jump between chapters
- Cover art that shows in the player
- Tags such as the title and the author
- An extension players recognise as an audiobook, which is why many of them save your listening position
M4B or MP3 for an audiobook?
MP3 audiobooks are usually a folder of files, one per chapter, with no chapter list built in. An M4B keeps the whole book in one file and carries its own chapter list, and AAC audio is usually smaller than MP3 at a similar quality.
MP3 still wins on reach: almost every device and player handles it. If you are not sure what your listeners use, offering both — an M4B and a ZIP of chapter MP3s — covers everyone.
How to play an M4B file
On a Mac, Apple's Books app plays M4B audiobooks, and they sync to an iPhone or iPad once they are in your library. On Windows, Android and Linux, VLC plays them, as do many dedicated audiobook apps.
If a player shows the book as one long track with no chapters, it is ignoring the chapter markers — the file is fine; try a player built for audiobooks.
M4B files and audiobook stores
Retail audiobook stores publish their own submission requirements, and these often ask for separate files per chapter rather than a single M4B. Check the specification of the store you plan to use before preparing files for it.
An M4B is ideal for distributing a book yourself — from your own website, to supporters, or as a bonus for readers. The M4B ToneCraft produces is meant for that kind of direct distribution and is not packaged for retail audiobook stores.
How to make an M4B
If you already have recorded chapters, command-line tools such as FFmpeg can join them into one AAC file and write the chapter markers; dedicated audiobook tools wrap the same steps in an interface.
If you are starting from the book itself, ToneCraft imports a DRM-free PDF, EPUB, DOCX, FB2, TXT, Markdown or HTML book of up to 100 MB, lets you review the chapters, narrates them, and exports an M4B with chapter markers, title and author tags and an optional cover, with loudness normalized (-18 LUFS by default) — or a ZIP of chapter MP3s.