Voxbound / M4B vs M4A vs MP3

M4B vs M4A vs MP3 for audiobooks: what's actually different

An M4B and an M4A are the same kind of file: MPEG-4 audio, usually AAC, with room for chapters, tags and a cover. The difference is how players treat them: Apple Books, for one, treats an .m4b as an audiobook and an .m4a as ordinary audio. MP3 is a different format, and an MP3 audiobook is often one file per chapter.

Updated 27 September 2026. The file checks on this page were run with FFmpeg 8.1 and Voxbound 4.2.0, and the claims about Apple Books link to their sources.

What's inside each

The official list of MP4 file types, kept by the MP4 registration authority, describes M4A as "iTunes MPEG-4 audio" and M4B as "iTunes AudioBook", each "protected or not" and able to contain a chapter track. A file can name one of those types in its header, but it doesn't have to match the extension: FFmpeg writes M4A for both, so a .m4b made with FFmpeg and the same book saved as .m4a differ only in their names.

Some files also carry an audiobook mark in their tags (media kind 2). Voxbound 4.2.0 sets it when it saves a book as .m4a, so players that read the mark file it as a book.

How to check a file

With FFmpeg installed, this shows the type named in the header, the audiobook mark if there is one, the audio codec and the chapters:

ffprobe -v error -show_entries format_tags=major_brand,media_type:stream=codec_name -show_chapters -of compact "book.m4b"

For a book Voxbound saved as .m4a, the last line ends with major_brand=M4A |tag:media_type=2: media_type=2 is the audiobook mark. No media_type means the file has no audiobook mark.

Renaming .m4a to .m4b

Renaming doesn't change anything inside the file; it changes how players that go by the extension treat it. Glenn Fleishman, writing in Six Colors, found that Books treated the .m4a version of his audiobook as plain audio and the .m4b version as an audiobook with chapters. So if Books won't import your audiobook, or treats it as an ordinary audio file, rename it:

Rename-Item "book.m4a" "book.m4b"

Then import it again: in Books on a Mac, choose File, then Import, as Apple's guide to adding audiobooks explains.

Renaming .m4b to .m4a

If a player doesn't list .m4b files but plays .m4a, rename the book:

Rename-Item "book.m4b" "book.m4a"

Nothing inside changes, so a player that opens .m4a files should open it too, unless it's copy-protected. A player that goes by the extension may treat it as ordinary audio rather than an audiobook. Saving the book as M4A in Voxbound 4.2.0 gives an .m4a with the audiobook mark set, for players that read it.

When renaming isn't enough

Renaming changes how a player treats one file. It can't fix what isn't in the file:

• If the book is several files, renaming doesn't join them. Combine them into one M4B first, as our guide to combining MP3 files into an M4B shows.
• If the file has no chapters, renaming doesn't add any. The check above lists none; our guide to chapters that don't show explains how that happens and how to make the book again with them.
• If the file is copy-protected, renaming doesn't make it play anywhere else, and neither the commands on these pages nor Voxbound can convert it.

Two claims you'll read elsewhere

Some pages say an M4A can't hold bookmarks. Whether a player remembers where you stopped depends on the player, not on the format.

Some pages say only M4B files can be copy-protected. The file type list above describes both as "protected or not", so the extension doesn't tell you. Voxbound can't convert protected files, whatever their extension.

Which to choose

• For Apple Books, Audiobookshelf or Prologue, use M4B. Our guide to the best M4B settings for each player covers the details.
• For a player that doesn't list .m4b files, use M4A.
• For a player or device that only plays MP3, use MP3, as one file per chapter if you want to move between chapters. Our M4B to MP3 guide shows both ways.

Voxbound saves a book in any of the three. As M4B or M4A it writes the chapters in both the QuickTime and Nero formats, with the tags and cover; as MP3 it writes the tags and cover but no chapters.

Questions

Short answers to what people ask most about the three formats.

What's the difference between M4A and M4B?

Inside, very little: both are MPEG-4 audio files, usually AAC, and both can hold chapters, tags and a cover. The difference is how players treat them: Apple Books, for one, treats an .m4b as an audiobook and an .m4a as ordinary audio.

Can I rename an M4A to M4B?

Yes. Renaming changes nothing inside the file, only how players that go by the extension treat it. It's the fix when Apple Books won't import your audiobook, or treats it as an ordinary audio file.

Do I need an M4A to M4B converter?

Not for one file: renaming .m4a to .m4b is all it takes, because nothing inside needs to change. A book in several files needs combining into one first.

How do I change an M4B to M4A?

Rename it from .m4b to .m4a. The audio, chapters and cover stay the same, but a player that goes by the extension may treat it as ordinary audio.

Is M4B better than MP3 for audiobooks?

For players that support it, yes: one M4B holds the whole book with chapters, a cover and tags. MP3 can carry chapter marks too, but not every player reads them, so an MP3 audiobook is often one file per chapter.

Do M4B files have DRM?

Only if they were sold with copy protection. The MP4 file type list describes both M4A and M4B as protected or not, so the extension doesn't tell you.