M4A and M4B: Same Container, Different Purpose
M4A and M4B both use the MPEG-4 container with AAC audio inside. The difference comes down to the file extension and a metadata flag. When a player sees .m4a, it treats the file as music. When it sees .m4b, it treats it as an audiobook. That means bookmarking, chapter navigation, and placement in your audiobook library instead of your music library.
Simply renaming .m4a to .m4b sometimes works for a single file, but you won’t get chapter markers. And if you have multiple M4A files that should be one book, renaming won’t combine them. Audiobook Binder Pro handles both: it merges your M4A files and outputs a proper M4B with chapters.
Where M4A Files Come From
Voice Memos. Every recording you make with Voice Memos on iPhone or Mac saves as M4A. Students recording lectures, journalists capturing interviews, and anyone dictating notes ends up with a collection of M4A files.
Apple ecosystem tools. GarageBand exports M4A. iTunes and Apple Music downloads are M4A. When you rip a CD through iTunes, you get M4A files by default.
Other sources. Many podcast apps download episodes as M4A. Audio editing tools like Ferrite Recording Studio on iPad export M4A. Screen recording audio tracks are often M4A.