How to create an M4B audiobook with AI (step by step)

Short answer: upload your manuscript to Warblize, choose a voice, then open Export, pick M4B (audiobook), review the book details and export. You get a single .m4b file narrated by AI, with a chapter marker for every chapter, your cover, and the title, author, narrator and series filled in. It plays in Apple Books, BookPlayer, Audiobookshelf, Plex and most audiobook apps.

Key facts

• M4B is the standard audiobook file: one file with the audio, chapter markers, cover art and book details.
• Warblize makes M4B audiobooks from EPUB, PDF, DOCX or TXT manuscripts, in 40+ languages.
• Each chapter of your book becomes a chapter marker you can skip to.
• Audio is AAC at 64 kbps, about 30 MB per hour, mastered so the average loudness is about −20 dB and peaks stay under −3 dB.
• Narration costs one credit per hour of finished audio. Exporting as M4B is free, and so is exporting again.

What is an M4B file?

An M4B is an audiobook in a single file. Inside it is the narration (AAC audio in an MP4 container) and everything a player needs to present it as a book:

• Chapter markers, so listeners can see the chapter list and jump between chapters.
• Cover art, shown in the player and on the lock screen.
• Book details: title, author, narrator, series, publisher, year, genre and description.
• An audiobook flag, so apps file it under Audiobooks instead of Music and remember where the listener stopped.

If you've ever been handed an audiobook as a folder of 40 MP3 files with names like track_07.mp3, M4B is the fix for that.

M4B vs MP3 vs WAV

Use M4B when people will listen to the file directly. Use MP3 when a store asks for it. Warblize exports all three from the same book, so you don't have to choose up front.

The old way to make an M4B

Making an M4B yourself from an ebook usually takes four or five tools and an afternoon:

1. Convert the ebook to text (for example with Calibre).
2. Run the text through a text-to-speech engine, often one robotic voice for everything.
3. Join the audio files and work out where each chapter starts.
4. Write the chapter markers and encode the M4B (m4b-tool or FFmpeg on the command line).
5. Add the cover and tags with another tool, and hope the player shows them.

Each step is a chance to lose chapter breaks, mangle the cover, or end up with one chapter far louder than the next.

How to create an M4B audiobook with Warblize

1. Upload your manuscript

Go to Warblize and upload your book as an EPUB, PDF, DOCX or TXT file, up to 100 MB. EPUB gives the best result: Warblize reads its table of contents for the chapters, and its cover, title, author, publisher, year, description and series for the book details. With a PDF, the bookmarks become chapters when there are any.

If your manuscript needs tidying first, see how to prepare a manuscript for an audiobook.

2. Check the chapters

Open the chapter list in the studio. Every chapter here becomes a chapter marker in your M4B, with its title, so make the list read the way you want listeners to see it. You can rename, reorder, add and remove chapters.

3. Choose the voices

Pick a narrator from 30 AI voices (hear them all here). For fiction with dialogue, press Find characters in the Characters panel: it works out who says every line and gives each character a matching voice. It's free, and it doesn't touch anything you've set by hand. See Find characters for how it works.

4. Listen and fix any line

Play a chapter. If a sentence sounds wrong, edit the text, regenerate it, or give it to a different voice. You can also shape how a character sounds and add emotion, such as a sigh or a whisper. Fixing a line before you export costs far less effort than fixing it after.

5. Choose M4B and review the book details

Open Export and set the format to M4B (audiobook). A Book details row appears; press Review (or just press Export, and the details open the first time). Check:

• Title (the only required field) and author.
• Narrator. Leave it empty to use the voice's name, for example "Charon (Warblize AI)".
• Series and number in series, so apps group books in order.
• Publisher, year, genre and description.
• Cover: upload a square image. 3200 × 3200 px is recommended, up to 10 MB. A cover that isn't square is cropped from the center, and the preview shows exactly what the M4B will get.

Anything you leave empty is either skipped or given a sensible default. Nothing optional is forced.

6. Export and download

Press Export. If any chapters haven't been narrated yet, Warblize generates them first; the export menu shows the credits needed before you start. It then masters every chapter to the same loudness and downloads a single .m4b file.

Open it in Apple Books, BookPlayer, Audiobookshelf, Plex or any audiobook player, and you'll see your cover, the book details and the full chapter list.

What's inside the M4B Warblize makes

What this saves you

• One upload instead of five tools. No Calibre, no command line, no tag editor.
• Chapters that stay chapters. Your table of contents becomes chapter markers automatically.
• A book that looks finished. Cover, author, narrator and series show up properly in listeners' apps.
• Even volume from start to end. Every chapter is mastered to the same loudness, so nobody reaches for the volume between chapters.
• More than one voice. A full cast with Find characters, not one flat text-to-speech voice.
• Changes without starting over. Fix a sentence, then export again. Re-exporting uses no credits.

Where to use your M4B

• Your own listening, or anyone you share it with: Apple Books, BookPlayer, Smart AudioBook Player, Audiobookshelf, Plex.
• Direct sales from your own website or shop, where you choose the format.
• Review and advance copies, as one tidy file instead of a folder.

Most retail stores and distributors ask for MP3 files instead, usually one per chapter; use MP3 with Chapters ZIP for those. Check the store's rules on AI narration first: ACX (Audible) doesn't accept it, while Spotify, Kobo and Google Play accept labeled digital narration. Where to sell an AI-narrated audiobook covers each store.

What it costs

Narration uses one credit per 60 minutes of finished audio. A 10-hour novel uses 10 credits, which is one month of the Creator plan ($29). Uploading, editing, finding characters and exporting as M4B cost nothing extra. To estimate your book, use the audiobook length calculator or see Warblize's current pricing.

Ready to make yours? Convert your EPUB to M4B or upload any manuscript.