Extract Audio from WMV to M4B — Free Online Tool
Extract audio from WMV video files and convert it to M4B format — the standard container for audiobooks and podcasts. This tool transcodes WMV's default WMA (wmav2) audio stream into AAC, wrapping it in an MPEG-4 container with chapter and bookmarking support ideal for long-form listening.
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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
WMV files store audio using Microsoft's proprietary WMA codec (wmav2 by default), which is incompatible with the M4B/MPEG-4 container. During this conversion, the video stream is completely discarded (-vn), and the WMA audio is decoded and re-encoded into AAC — the native codec for M4B. The resulting AAC stream is then written into an MPEG-4 container with the +faststart flag applied, which repositions the metadata atom at the start of the file for efficient streaming and playback. Because M4B supports chapters and bookmarking, the output file is immediately usable in audiobook players like Apple Books or podcast apps, even if the source WMV had no chapter data.
What Each Flag Does
| Flag | What it does |
|---|---|
ffmpeg
|
Invokes the FFmpeg program. In the browser-based version of this tool, this runs via FFmpeg.wasm (WebAssembly), so no files leave your device. |
-i input.wmv
|
Specifies the input file — your WMV source in the ASF container, typically containing an msmpeg4 video stream and a wmav2 audio stream. |
-vn
|
Disables video output entirely, discarding the WMV's video stream. Since M4B is a pure audio format, this flag ensures no video data is processed or written, which also speeds up the conversion significantly. |
-c:a aac
|
Transcodes the audio stream using FFmpeg's built-in AAC encoder. This is necessary because WMA (wmav2), the default WMV audio codec, is not compatible with the MPEG-4 container — the WMA audio must be fully decoded and re-encoded as AAC. |
-b:a 128k
|
Sets the AAC audio bitrate to 128 kilobits per second, which is the default for M4B output. This is a reasonable balance between file size and quality for speech-heavy audiobook content; increase to 192k or 256k if the source WMV had high-quality music or audio. |
-movflags +faststart
|
Moves the MPEG-4 container's metadata index (moov atom) to the beginning of the output file. This is required for the M4B format to stream or begin playback efficiently on Apple devices and podcast apps before the full file is loaded. |
output.m4b
|
The output filename with the .m4b extension. FFmpeg uses this extension to write an MPEG-4 container branded as M4B, which tells Apple Books, podcast clients, and other audiobook players to enable chapter navigation and bookmarking features. |
Common Use Cases
- Convert a WMV recording of a lecture, sermon, or conference talk into an M4B audiobook file that listeners can pause and resume with automatic bookmarking on iPhone or iPad
- Strip audio from a Windows Media Video training course and repackage it as an M4B podcast episode for distribution through Apple Podcasts or compatible RSS feeds
- Archive WMV audio commentary tracks — such as DVD extras ripped in WMV format — into M4B files that preserve long-form content with resumable playback
- Convert a WMV recording of an audiobook or spoken-word performance you own into M4B so it appears correctly categorized in Apple Books alongside purchased audiobooks
- Extract the audio from a WMV screen recording or webinar to create a hands-free listening version in a format optimized for mobile podcast apps with chapter navigation support
- Migrate a personal library of WMV audio content created in Windows Media Player into M4B format for playback on non-Windows devices that do not support WMA natively
Frequently Asked Questions
Yes, there is a small quality loss because this is a transcode between two lossy codecs — WMA (wmav2) and AAC. The audio is fully decoded from WMA first, then re-encoded to AAC at 128k, which introduces a second generation of lossy compression. At 128k AAC the result is generally transparent for speech and acceptable for most music, but if the source WMV already had low-bitrate WMA audio (e.g. 64k), the degradation may be more noticeable. If quality is critical, use a higher -b:a value in the FFmpeg command.
M4B supports chapters natively, but this tool does not automatically generate chapter markers from a WMV file that lacks them, since WMV does not support chapter data. The output M4B will be a single continuous audio track with no chapter points. If you want to add chapters, you would need a second step using a tool like mp4chaps or an audio editor after the conversion is complete.
Both M4B and M4A use the same MPEG-4 container with AAC audio — the difference is purely in the file extension and the 'brand' marker inside the container. The .m4b extension signals to Apple devices and podcast apps that the file is an audiobook or podcast, enabling automatic bookmarking and categorization in Apple Books. Using .m4a would produce an otherwise identical file but it would be treated as regular music, losing the resumable-playback behavior that makes M4B useful for long-form audio.
The -movflags +faststart flag causes FFmpeg to move the MPEG-4 moov atom (which contains the file's metadata and index) to the beginning of the file after encoding. By default, FFmpeg writes the moov atom at the end. Placing it at the front allows players and streaming services to begin playback before the entire file has downloaded, which is especially beneficial for large M4B audiobook files. It has no effect on audio quality or file size.
Replace the -b:a 128k value in the command with a higher bitrate. For example, use -b:a 192k or -b:a 256k for noticeably better AAC quality, especially if your source WMV had high-quality WMA audio above 192k. For spoken-word audiobook content, 64k AAC is often sufficient and produces much smaller files. The full adjusted command would look like: ffmpeg -i input.wmv -vn -c:a aac -b:a 192k -movflags +faststart output.m4b
The command shown processes one file at a time, but you can batch process with a simple shell loop. On Linux or macOS, run: for f in *.wmv; do ffmpeg -i "$f" -vn -c:a aac -b:a 128k -movflags +faststart "${f%.wmv}.m4b"; done. On Windows Command Prompt, use: for %f in (*.wmv) do ffmpeg -i "%f" -vn -c:a aac -b:a 128k -movflags +faststart "%~nf.m4b". This is particularly useful for migrating an entire WMV media library to M4B.
Technical Notes
WMV files use the Advanced Systems Format (ASF) container, and their audio is almost always encoded with Microsoft's WMA codec (wmav2), which is not supported by the MPEG-4 container used by M4B. This means a full audio transcode — not a remux — is always required for this conversion; there is no lossless path. The -vn flag ensures all video data from the WMV is discarded and never processed, making the conversion faster than a full video transcode. M4B does not support multiple audio tracks, so if the source WMV contained more than one audio stream (WMV supports this), only the first stream is extracted by default. If you need a specific track, add -map 0:a:1 (or the appropriate index) to the command. ID3-style metadata tags embedded in the WMV (such as title and artist) may not transfer automatically because WMV stores metadata in ASF-specific headers; you may need to add metadata manually using -metadata title="Title" flags or a tag editor after conversion. DRM-protected WMV files cannot be processed by FFmpeg or this tool regardless of bitrate settings.