Extract Audio from MP4 to M4B — Free Online Tool
Extract audio from an MP4 video and save it as an M4B audiobook file, preserving AAC audio with chapter markers intact. This is the ideal conversion for turning recorded lectures, long-form video content, or video podcasts into a bookmarkable audiobook format playable in Apple Books, Overcast, and compatible podcast apps.
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FFmpeg Command
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Note: Browser-based encoding uses approximate quality targets. For precise CRF compression, copy the FFmpeg command above and run it on your desktop.
Estimated output:
Conversion Complete!
DownloadHow It Works
During this conversion, FFmpeg strips the video stream entirely from the MP4 container and repackages the AAC audio track into an M4B file. If the source MP4 already uses AAC audio (the most common default), the audio is passed through without re-encoding, preserving the original quality bit-for-bit. If the MP4 contains MP3 or Opus audio instead, FFmpeg will transcode it to AAC at the specified bitrate. M4B is structurally an MPEG-4 audio container — essentially an AAC audio file with audiobook-specific metadata support — so the container swap is lightweight. Any chapter metadata embedded in the original MP4 is carried over into the M4B, enabling chapter navigation in audiobook players. The -movflags +faststart flag reorganizes the file's metadata to the front, allowing the M4B to begin streaming or playing before it is fully downloaded.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg program. This is the same engine running in your browser via WebAssembly — copying and running this command locally will produce an identical output file. |
-i input.mp4
|
Specifies the source MP4 file as the input. FFmpeg will read the video and audio streams from this container, though only the audio will be used in the output. |
-vn
|
Disables video output entirely — this is essential for M4B conversion since the format is audio-only and does not support video streams. Without this flag, FFmpeg would attempt to include the video track and fail or produce a broken file. |
-c:a aac
|
Sets the audio codec to AAC (Advanced Audio Coding), which is the required codec for M4B files. If the source MP4 already contains AAC audio, FFmpeg may perform a stream copy; if the source uses a different codec like Opus or MP3, it will transcode to AAC. |
-b:a 128k
|
Sets the AAC audio bitrate to 128 kilobits per second, a standard quality level suitable for most content including speech and music. For spoken-word audiobooks, you can reduce this to 64k without noticeable quality loss, significantly reducing file size. |
-movflags +faststart
|
Moves the M4B file's metadata index (MOOV atom) to the beginning of the file after encoding. This allows the audiobook to begin playing before it is fully downloaded, which is important for streaming M4B files from cloud storage or audiobook servers. |
output.m4b
|
Specifies the output filename with the .m4b extension, which is what signals to Apple Books, Overcast, and other audiobook apps that this file should be treated as bookmarkable long-form audio content rather than a standard music track. |
Common Use Cases
- Convert a recorded university lecture series saved as MP4 files into M4B audiobooks so you can listen on your iPhone with bookmarking, resuming exactly where you left off between commutes
- Turn a long-form video podcast or YouTube download into an M4B file for import into Apple Podcasts or Overcast, gaining chapter navigation support if the original MP4 had chapter markers
- Extract audio from a professionally recorded audiobook that was distributed as a video file (common on some learning platforms) and repackage it into the standard M4B format for your audiobook library
- Convert a multi-hour conference keynote or documentary MP4 into an M4B so you can consume the content on a long flight using an audiobook app with sleep timer and speed control features
- Strip video from a language-learning MP4 course and save as M4B to add it to your audiobook player alongside other study materials, with chapters separating each lesson
Frequently Asked Questions
If your MP4 already contains AAC audio — which is the default codec for most MP4 files — FFmpeg will remux the audio stream directly into the M4B container without any re-encoding, meaning zero additional quality loss. However, if your MP4 uses a different audio codec such as Opus or MP3, FFmpeg must transcode it to AAC, which will introduce a small amount of quality change. You can check your source file's audio codec using a tool like MediaInfo before converting.
Yes. FFmpeg copies chapter metadata from the MP4 to the M4B container during this conversion. M4B has native support for chapters, and apps like Apple Books, Overcast, and Pocket Casts will display and navigate them. However, if your original MP4 has no embedded chapter markers, the resulting M4B will simply play as a single continuous audio track — chapters do not get created automatically from the video.
M4B and M4A are both AAC audio in an MPEG-4 container, but the M4B file extension signals to audiobook and podcast apps that the file should be treated as long-form bookmarkable content rather than music. Apple Books and Overcast, for example, use the .m4b extension to enable features like automatic bookmarking, playback speed adjustment, and chapter navigation. Renaming an M4A to M4B works in many cases, but converting properly via FFmpeg also ensures the container flags are correctly set.
Replace the -b:a 128k value in the command with a lower bitrate such as -b:a 64k or -b:a 96k. For spoken-word audiobook content, 64k AAC is generally indistinguishable from higher bitrates and will roughly halve the file size compared to 128k. The full modified command would be: ffmpeg -i input.mp4 -vn -c:a aac -b:a 64k -movflags +faststart output.m4b. Avoid going below 64k for AAC as speech clarity can degrade noticeably.
Yes. On Linux or macOS, you can run a shell loop: for f in *.mp4; do ffmpeg -i "$f" -vn -c:a aac -b:a 128k -movflags +faststart "${f%.mp4}.m4b"; done. On Windows using PowerShell, use: Get-ChildItem *.mp4 | ForEach-Object { ffmpeg -i $_.FullName -vn -c:a aac -b:a 128k -movflags +faststart ($_.BaseName + '.m4b') }. This is especially useful for lecture series or multi-part audiobooks where you need to process many files at once.
M4B compatibility on Android depends on the app. Apps like Voice Audiobook Player, Smart AudioBook Player, and Bound all support M4B natively including chapters and bookmarking. The built-in Android music player may not handle M4B correctly or may ignore chapter data. If you need guaranteed broad Android compatibility, consider converting to MP3 instead, though you would lose chapter support. For iPhone and iPad with Apple Books, M4B is the native preferred format.
Technical Notes
M4B is technically an AAC audio stream inside an MPEG-4 Part 14 container — the same underlying structure as MP4 and M4A — but distinguished by its extension and intended use as an audiobook format. Because MP4 and M4B share the same container spec, the conversion is extremely efficient when the source audio is already AAC: FFmpeg performs a stream copy (-c:a aac without re-encoding when the codec matches) rather than a full decode-encode cycle. The -vn flag is critical here — without it, FFmpeg would attempt to process the video stream, which M4B does not support, causing an error or a failed output. The -movflags +faststart flag moves the MOOV atom (the file's index/metadata block) to the beginning of the file, which is particularly useful if the M4B will be hosted for streaming rather than local playback. One important limitation: M4B supports only a single audio track, so if your source MP4 contains multiple audio tracks (e.g., multiple language dubs), only the first audio track will be included in the M4B output. Subtitle tracks from the MP4 are also dropped, as M4B does not support subtitles. ID3-style tags such as title, artist, and album metadata embedded in the MP4 are generally preserved in the output M4B and will display in audiobook apps.