What changes, and what doesn’t
| You drop | What happens | Quality |
|---|---|---|
| M4A, AAC, MP4 audio | Audio copied into an M4B | Identical, lossless |
| FLAC, WAV (lossless) | Converted to AAC | Excellent at any setting |
| MP3, OGG, Opus | Converted to AAC | Keep the bitrate close to the original |
Format by format
M4A to M4B
M4A and M4B are the same kind of file, MPEG-4 with AAC audio, so the audio is copied across without re-encoding: lossless, and done in seconds even for a long book. Chapters, tags and the cover come with it. The new extension tells Apple Books and audiobook apps to treat it as a book.
MP4 to M4B
An MP4 video with an AAC soundtrack (a recorded talk, a lecture, a video audiobook) gives up its audio track, copied without re-encoding; the picture is left behind. Other soundtracks are converted.
AAC to M4B
A bare .aac file has the right audio but no container for chapters or a cover. It’s wrapped in an M4B as it is.
FLAC and WAV to M4B
Lossless sources, from a CD rip or a recording session. They’re encoded to AAC once, so any quality setting sounds as good as AAC can at that bitrate: 64 kbps mono is clear for narration, at about 29 MB an hour, a fraction of the original size.
OGG, Opus and WMA to M4B
These are already compressed, so converting is lossy a second time. Pick a bitrate at or a little above the original to keep what quality there is. Opus is excellent at low bitrates, so an Opus book may grow when converted to AAC at the same sound quality.
MP3 to M4B
One MP3 becomes one M4B here. For a book in many MP3s, the MP3 to M4B maker binds them into one file with a chapter per file.
Making one audiobook from many files? The MP3 to M4B maker joins them with a chapter per file. To set the cover and tags afterwards, use the metadata editor.