Audiobook File Formats Explained: MP3, M4B, WAV and AAX
Audiobooks come in several file formats, and the right one depends on whether you are producing, uploading or listening. This guide explains the common audiobook file types, what each one is for, and exactly what format to deliver to ACX and other distributors.
Quick answer: what format are audiobooks in?
For producing and archiving, audiobooks are recorded and edited as uncompressed WAV files. For uploading to ACX (Audible), each chapter is exported as an MP3 at 192 kbps or higher, constant bit rate, 44.1 kHz. Listeners usually get AAX (Audible), M4B (Apple Books and many apps) or plain MP3, depending on the store.
Common audiobook file types
| Format | What it is | Best used for |
|---|---|---|
| WAV | Uncompressed audio, full quality, large files | Recording, editing and archive masters |
| MP3 | Compressed audio, plays almost everywhere | Retailer uploads (ACX), simple listening |
| M4B | AAC audio in an MP4 container with chapter markers and bookmarks | Single-file audiobooks for Apple Books and audiobook apps |
| AAX / AAXC | Audible’s protected (DRM) format | What Audible delivers to listeners — not something you produce |
| FLAC | Lossless compressed audio | High-quality archives; accepted by some distributors |
WAV: your production master
Record and edit in WAV at 44.1 kHz or 48 kHz, 16-bit or 24-bit. WAV keeps every detail, so editing, noise reduction and mastering don’t stack up compression artifacts. Keep your final mastered WAVs as an archive — you can always make new MP3s or M4Bs from them later.
MP3: the upload format for ACX
ACX requires one MP3 per chapter or section, encoded at 192 kbps or higher, constant bit rate (CBR), 44.1 kHz, with every file in the same mono or stereo format. Variable bit rate (VBR) files are rejected. See our full ACX audio requirements checklist.
M4B: the single-file audiobook
M4B packs a whole book into one file with chapter markers, so listeners can jump between chapters and keep their place. It is the native audiobook format for Apple Books and works in many audiobook players. If you sell audiobooks directly from your own website, M4B gives buyers the best listening experience.
AAX: Audible’s listener format
When someone buys your book on Audible, they download it in Audible’s protected AAX/AAXC format. You never create these files yourself — Audible converts the MP3s you upload through ACX.
Mono or stereo?
Most spoken-word audiobooks are delivered in mono. It halves file size with no loss for a single voice. Stereo is useful when the production includes music or sound design. Whatever you choose, every file must match.
File naming and structure
- One file per chapter or section, plus separate opening and closing credits.
- Name files in order: 00_Opening_Credits, 01_Chapter_1, 02_Chapter_2 … 99_Closing_Credits.
- Keep each file under 120 minutes; split very long chapters into parts.
- Add ID3 tags (title, author, narrator, chapter) to every MP3.
What we deliver
Every mastering project includes ACX-ready MP3s for upload and full-quality WAV masters for your archive. A single-file M4B with chapter markers is available as an add-on.
Want your audiobook ACX-ready without the guesswork?
Not sure your files are in the right format? Send us a chapter and we’ll check it, master it and return ACX-ready files.
