MP4 to M4A

Why this takes seconds, and comes out identical

An MP4 is a container: a wrapper holding a picture stream and a sound stream next to each other. The sound stream is AAC. An .m4a file is the same container format holding the AAC stream on its own. So there is nothing to convert — the stream is copied across and a new wrapper is written around it.

The result is measurably identical. On this site's test suite, the AAC stream extracted from an MP4 and the AAC stream inside the source file match byte for byte, and a plan that claims a copy is failed by the build if it does not. What is not identical is the file as a whole: FFmpeg writes its own container metadata — its encoder tag, its timestamps — so the wrapper differs while every byte of audio does not. That is the honest version of the claim, and it is the one you can check.

M4A versus MP3, in practice

M4A plays on every iPhone, every Mac, every Android phone, Windows 10 and 11, every modern browser and almost every car made in the last decade. MP3 plays on all of those plus the genuinely old ones — a 2006 car stereo, a hospital dictation machine, an upload form that checks the extension.

So: M4A when the file is for you or for a modern device, and it costs nothing. MP3 when something on the other end insists, and accept the one re-encode. What is never worth doing is converting an MP4 to MP3 “for quality” — it is the one choice that is strictly worse in both size and fidelity.

When the copy is not available

Three cases. If you fold the audio to mono, the samples themselves change, so it has to be re-encoded — the panel switches its wording as soon as you press it. If you deliberately choose a bitrate lower than the source, you are asking for a smaller file, so a copy would not be doing the job. And if the video's soundtrack is not AAC — Opus in a WebM, AC-3 in an MKV, PCM in a camera master — then M4A means encoding to AAC, which the panel will tell you plainly.