M4A is an MPEG-4 container holding AAC audio, which is what a voice memo and a music store purchase both are. WAV is a thin container around uncompressed PCM samples, which is why a three minute song weighs thirty megabytes. ffmpeg sits between the two, and this page says exactly what happens to an M4A file on the way to becoming a WAV one.
What runs when an M4A file becomes WAV
ffmpeg held to a single thread, on a machine we rent and watch. Your M4A file is uploaded once, ffmpeg runs once, the WAV comes back, and neither file is kept. M4A to WAV is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the M4A into the WAV
The running time and the channel layout. A stereo M4A comes back as a stereo WAV of the same length: this is a re-encode, not an edit, and nothing here normalises, trims or boosts the M4A.
Everything the M4A still had. ffmpeg decodes the M4A in full before it writes a single byte of WAV, so nothing is skipped on the way in, whatever was already missing.
What M4A to WAV costs you
Nothing further, and nothing recovered either. WAV stores every sample it is handed faithfully, but the samples it is handed came out of an M4A decoder. The WAV is an exact copy of an approximation, and it is several times the size of the M4A for the same sound.
Album art, where the M4A carried any. ffmpeg passes -vn on every audio target here, which counts an embedded cover as a video stream and drops it along with the film it was never part of. That is deliberate rather than careless: keeping it makes the mux fail outright on several audio containers, and hiding the choice would be lying by omission.
Any hope of WAV sounding better than the M4A. Nothing in this pass adds information, and a bitrate higher than the M4A deserved only makes a larger file of the same recording.
If a model is going to read the WAV
A WAV file is a waveform, and a language model reads text, so handing an assistant this WAV directly gets you nothing. The WAV becomes useful the moment somebody turns the sound into words, and that is a separate job with its own page here. M4A to WAV is the step before it: get the sound out of the M4A in a container the transcriber accepts, then hand the model a transcript instead of a file it cannot open.
Running M4A to WAV against the real engine with a real file, ffmpeg wrote 266,318 bytes of WAV in 0.1 seconds, machine otherwise idle. That is one M4A file on one day and not an average, which is why the number is given together with the file that produced it.
The M4A to WAV verdict was reached by ffprobe, meaning the container was actually demultiplexed, recognised under the right name, and carried at least one stream. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the WAV file has anything inside it.
The same M4A file, sent somewhere else
Other outputs the probe measured out of an M4A file, so the cost of choosing WAV can be read against something. Sizes do not compare across engines, because each family was probed with its own M4A witness file.
M4A to MP3: 24,776 bytes in 0.1 seconds, verified by ffprobe.
M4A to FLAC: 156,945 bytes in 0.1 seconds, verified by ffprobe.
M4A to OGG: 9,182 bytes in 0.1 seconds, verified by ffprobe.
M4A to OPUS: 27,860 bytes in 0.1 seconds, verified by ffprobe.
M4A to AAC: 27,317 bytes in 0.2 seconds, verified by ffprobe.
M4A to WMA: 55,094 bytes in 0.1 seconds, verified by ffprobe.
Other ways into WAV, and what they measured
Among the published routes into WAV, M4A is the eighth largest output of the 16 measured. The witness files differ, so this ranks the probe run and not your document.
MKV to WAV: 265,422 bytes in 0.1 seconds.
MOV to WAV: 266,318 bytes in 0.1 seconds.
MPG to WAV: 265,038 bytes in 0.1 seconds.
OGG to WAV: 264,678 bytes in 0.1 seconds.
OPUS to WAV: 288,078 bytes in 0.1 seconds.
What we will not pretend about M4A to WAV
An M4A file over 150 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An M4A to WAV run that passes 240 seconds is killed, and the ffmpeg process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
The M4A file behind the measurement on this page was a 3 second sine wave at 440 hertz, built by ffmpeg for the probe itself. That is why M4A to WAV came out at 0.1 seconds, and it is not what your file will do. The measurement proves the pair works and says almost nothing about how long yours will take.
M4A to WAV costs half a credit per conversion, whatever the length of the M4A. That is the same as an image or a document here, and it is a measurement rather than a gesture: an audio target on this engine is done in under two seconds, where a video target can take minutes.
An M4A longer than 60 minutes is refused before encoding. ffprobe measures the M4A first and answers with a refusal, rather than starting a job the 240 second guard would kill halfway to the WAV.
M4A to WAV waits at the audio door, which is 2 slots wide and gives up after 5 seconds rather than making you wait twenty times the length of the work. Until the 30th there was one door for everything, and a single video transcode starved every audio conversion for its whole duration: a WAV that needed less than a second was answered with a refusal after twenty.
Nothing here reads what is inside the M4A. No transcription, no scene detection, no summary: M4A to WAV is a container and codec operation, and the WAV says exactly what the M4A said, in a different wrapper. The sound is the whole payload.
One M4A file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
M4A to WAV: what people ask
What actually converts my M4A file to WAV?
ffmpeg does it, held to a single thread, on a machine we rent and watch. Not a browser trick and not somebody else service: the M4A file is uploaded once, ffmpeg runs once, the WAV comes back, and neither file is kept afterwards.
How long does M4A to WAV take?
The probe measured 0.1 seconds for 266,318 bytes of WAV, on a 3 second file. An audio target scales far better than a video one: three minutes of source through this engine takes under a second, because only the sound is encoded. Past 240 seconds any run is stopped, and an audio one does not get close.
What do I lose going from M4A to WAV?
The one to know about first: Nothing further, and nothing recovered either. WAV stores every sample it is handed faithfully, but the samples it is handed came out of an M4A decoder. The WAV is an exact copy of an approximation, and it is several times the size of the M4A for the same sound.
Will the WAV sound better than the M4A?
No. Nothing in this pass adds information to the M4A, so the WAV is at best an exact carry of what you sent. Asking for a higher bitrate than the M4A deserved just makes a bigger file of the same recording.
Is M4A to WAV free?
There is a free allowance every month, and one M4A to WAV conversion costs half a credit against it however long the M4A runs. An audio target is charged per conversion and not per minute, because the engine encodes three minutes of sound in well under a second.