What are these three formats?

WMA is the legacy Windows Media audio format, M4B is an MP4 container used for audiobooks and normally holds AAC audio, and CAF is Apple's Core Audio Format, which is usually a container for PCM. All three are awkward in a browser, which is why they are handled by a bundled decoder rather than the in page path.

Why only one file at a time?

It is a deliberate choice rather than a missing feature. These sources are rare, and reporting a failure clearly matters more here than processing a batch, because the failure mode for an unusual variant is a container quirk rather than a simple error. One file per run keeps that reporting honest.

My audiobook has cover art. Will that break the conversion?

No, it is handled. An attached picture in an M4B is an extra stream, and the decode arguments exclude video and image streams so that the cover art cannot confuse the output mapping. The audio comes through and the metadata that the target can hold is preserved.

Why did an m4a file fail?

Because this tool is aimed at the three listed families and the encoder path is chosen for them. A file that claims to be one thing and contains another is only discovered when the run fails, and the message says the file may be unsupported, protected or corrupt. The ALAC converter handles the lossless Apple case.

Is there a quality setting?

No. The targets are fixed: MP3 at 192 kilobits per second, WAV at 16 bit PCM, and FLAC at its own lossless setting. That keeps the tool about getting a file into a usable form. If you need a different bitrate or bit depth, decode here first and then run the universal converter on the result.

WMA/M4B/CAF Decoder

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WMA/M4B/CAF Decoder turns three awkward containers into something every player understands. Point it at a Windows Media file, an audiobook or a Core Audio Format file and get back an MP3, a WAV or a FLAC.

What is WMA/M4B/CAF Decoder?

Drop a single file into the panel, choose one of three output targets and press decode. A progress bar tracks the run and the finished file appears as a row with its name, its size in kilobytes and a download control.

The panel and the three targets:

A drop zone accepting files with the wma, m4b, caf and asf extensions, with drag and drop or a click to browse.
Three output buttons: MP3 for universal playback, WAV for lossless and uncompressed, and FLAC for lossless and compressed.
A progress bar with a percentage readout while the decode runs.
A result row naming the new file, its size and a download control, plus a clear button to reset the panel.
A single file per run, deliberately, because these sources are rare enough that a queue would only complicate the error reporting.

Each target maps to a specific encoder: MP3 is written at 192 kilobits per second, WAV as 16 bit little endian PCM, and FLAC with its own lossless encoder. All three runs decode through the bundled encoder core rather than the in page path, because none of these containers can be handled natively. Every run is a genuine transcode rather than a rename, because the audio is decoded and re-encoded into the target; a protected file cannot be handled by changing its extension, and the three targets each map to a specific encoder. The arguments carry two deliberate guards. First, video and image streams are excluded, which matters most for audiobooks: an M4B file usually carries cover art as an attached picture stream, and without the guard that stream confuses the output mapping. Second, metadata is preserved where the source has it. The remaining limitation is the source itself: Core Audio Format files can hold several PCM variants, and one that the core cannot read fails with a message that says the file may be unsupported, protected or corrupt rather than pretending the run succeeded.

What it cannot do:

Protected files are not decoded, and a message saying so is the only outcome available for them.
Only one file is handled per run, so a library of audiobooks means repeating the process.
The MP3 target is fixed at 192 kilobits per second and the WAV target at 16 bit, so there are no quality choices.
It decodes to the three listed targets only; anything else needs a second step through the universal converter.
Excessively large files are limited by the memory the browser will give the encoder core.
Nothing is analysed first, so an unusual variant inside a supported container is only discovered when the run fails.

Use it when a file from one of these three families will not open anywhere modern: an old Windows Media recording, an audiobook that needs to play on a device without M4B support, or a Core Audio Format file exported from a Mac application. WAV is the safe choice when the result will be edited, and MP3 when it needs to play everywhere.

How to use WMA/M4B/CAF Decoder

Drop a file with a wma, m4b, caf or asf extension into the panel, or click to browse for one.
Choose the output target: MP3 for universal playback, WAV for lossless and uncompressed, or FLAC for lossless and compressed.
Press the decode button and watch the progress bar, remembering the encoder core may load first.
Download the finished file from the result row, or clear the panel to start another.

When to use WMA/M4B/CAF Decoder vs related tools

WMA/M4B/CAF Decoder is the rescue tool for containers that most browsers and devices will not open, so it belongs before any other processing. ALAC/M4A Lossless Converter covers the Apple lossless side of the same problem, which is a distinct codec inside a similar container. Once the file is in a normal format, Audio Format Converter handles the usual conversion work, and Audio Trimmer is the natural next step for cutting a long audiobook or recording down. To write a WMA file rather than read one, WMA Converter creates that container directly.

Privacy & Security

This tool runs entirely in your browser — no data ever leaves your device. There is no server round-trip, no upload, no logging, and no account required. Your input is processed locally using client-side JavaScript and is never stored, transmitted, or accessible to anyone else. When you close the tab, everything disappears.