MyMediaTools

Convert WMV to M4B — Free Online Tool

Convert WMV video files to M4B audiobook format by extracting and re-encoding the audio track to AAC inside an MPEG-4 container with chapter and bookmarking support. This is ideal for turning recorded lectures, video courses, or spoken-word WMV content into a portable audiobook file compatible with Apple Books, iPhone, and podcast apps.

FFmpeg Command

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Estimated output:

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How It Works

WMV files store audio using Windows Media Audio (WMA/wmav2) inside Microsoft's Advanced Systems Format (ASF) container. M4B is a pure audio format — it has no video track — so this conversion discards the video stream entirely and extracts only the audio. That audio is then re-encoded from wmav2 (or whatever audio codec the WMV uses) into AAC, which is the native audio codec for the MPEG-4 container family. The resulting M4B file gains the ability to store chapter markers and playback position bookmarks, features the ASF container does not support. The -movflags +faststart flag reorganizes the file's internal metadata to the front of the file, which enables progressive playback in streaming-capable players.

What Each Flag Does

Flag What it does
ffmpeg Invokes the FFmpeg multimedia processing tool. In this browser-based tool, FFmpeg runs locally via WebAssembly (FFmpeg.wasm) — your WMV file never leaves your device.
-i input.wmv Specifies the input WMV file. FFmpeg reads the ASF container and identifies the available streams — typically an msmpeg4 or msmpeg4v2 video stream and a wmav2 audio stream — before applying the conversion settings.
-c:a aac Sets the audio codec to AAC (Advanced Audio Coding), which is the required audio format for M4B files. Since WMV uses wmav2 audio, a full transcode is performed — the audio is fully decoded from wmav2 and re-encoded as AAC. The video stream is automatically dropped because M4B has no video codec support.
-b:a 128k Sets the AAC audio output bitrate to 128 kilobits per second. This is the standard default for spoken-word M4B audiobooks, providing a good balance between file size and audio clarity for voice content typical in converted WMV lectures or recordings.
-movflags +faststart Relocates the MPEG-4 container's metadata index (the moov atom) to the beginning of the output M4B file. This is a best practice for M4B files as it allows audiobook players and podcast apps to begin playback before the entire file has been downloaded or buffered.
output.m4b Specifies the output filename with the .m4b extension, which signals to Apple Books, iOS, and other audiobook-aware players that this file supports bookmarking and chapter navigation — distinguishing it from a generic .m4a audio file with identical internal structure.

Common Use Cases

  • Convert a recorded university lecture or webinar saved as WMV into an M4B audiobook so you can listen on your iPhone with automatic resume-where-you-left-off bookmarking
  • Transform a Windows Media Player-based e-learning course into an M4B file compatible with Apple Books, enabling chapter navigation between course modules
  • Extract and repackage the audio narration from a WMV corporate training video into an M4B podcast episode for internal distribution via podcast apps
  • Convert an old WMV audiobook rip — common in the early 2000s Windows Media era — into a modern M4B file playable on contemporary iOS and macOS devices without Windows Media Player
  • Archive spoken-word WMV content (interviews, radio shows, sermons) as M4B files with chapter support for easier long-form navigation
  • Prepare WMV-format language learning videos for offline listening on an iPod or iPhone by stripping the video and producing a lightweight AAC-based M4B file

Frequently Asked Questions

Yes, some quality loss is unavoidable because this is a transcode between two lossy codecs — wmav2 (used in WMV) to AAC (used in M4B). Each re-encoding step introduces generation loss. However, at the default bitrate of 128k AAC, the result is perceptually transparent for spoken-word content like audiobooks and lectures, which is the primary use case for M4B. If the source WMV uses a high audio bitrate and contains music, increasing the output bitrate to 192k or 256k will help preserve fidelity.
The M4B container itself supports chapters, but the FFmpeg command as shown will not automatically generate chapter markers from the WMV source, because WMV/ASF does not carry chapter metadata that maps directly to M4B chapters. The output M4B file will be a single continuous audio track. To add chapters, you would need to supply a chapter metadata file separately using FFmpeg's -i flag with a metadata file, or use a tool like mp4chaps after conversion.
M4B is a strictly audio-only format — it does not support video streams. The format exists specifically for audiobooks and podcasts, and no M4B-compatible player (Apple Books, Overcast, Podcast Addict) expects or renders video. The FFmpeg command omits any -c:v flag and does not map the video stream, so it is silently discarded. This is the correct and intended behavior for this conversion.
Basic text metadata such as title and artist stored in the ASF container will often be carried over by FFmpeg into the M4B's ID3-compatible atom tags. However, embedded album art and DRM-protected WMV files (a common feature of Windows Media content) will not transfer correctly. DRM-locked WMV files cannot be decoded or converted by FFmpeg at all — the source file must be a DRM-free WMV for this conversion to succeed.
Adjust the value after -b:a to change the AAC audio bitrate. For example, replace 128k with 192k for higher quality (useful if the source WMV had high-bitrate music audio) or 96k to produce a smaller file (acceptable for voice-only spoken-word content). The supported options are 64k, 96k, 128k, 192k, 256k, and 320k. For typical audiobook narration, 96k or 128k AAC is indistinguishable from higher bitrates.
Yes. On Linux or macOS, you can run: for f in *.wmv; do ffmpeg -i "$f" -c:a aac -b:a 128k -movflags +faststart "${f%.wmv}.m4b"; done. On Windows Command Prompt, use: for %f in (*.wmv) do ffmpeg -i "%f" -c:a aac -b:a 128k -movflags +faststart "%~nf.m4b". This processes every WMV file in the current directory and outputs a corresponding M4B file with the same base filename.

Technical Notes

WMV files use the ASF (Advanced Systems Format) container, which is a Microsoft proprietary format designed for streaming. The audio codec in WMV is almost always wmav2, which is not natively supported in the MPEG-4 container family — this is why re-encoding to AAC is mandatory rather than a simple remux. The M4B format is technically an MPEG-4 Part 14 container (like MP4 and M4A) with a different file extension that signals audiobook intent to compatible players. The -movflags +faststart flag is particularly important for M4B files that may be streamed over a network or downloaded progressively, as it moves the moov atom (the file's index) from the end to the beginning of the file. One important limitation: if your WMV file was protected with Windows Media DRM (Digital Rights Management), FFmpeg cannot read or decode the audio, and the conversion will fail with a decryption error. Only DRM-free WMV files are compatible with this tool. Additionally, WMV files sometimes contain multiple audio tracks (e.g., different language dubs), but M4B supports only a single audio track — FFmpeg will select the first (default) audio stream from the WMV automatically.

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