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M4A is one of the clearest examples of a problem that runs through audio and video generally: the extension names the box, not the contents. Two files both ending in .m4a can be as different inside as a photograph and a scan.
An M4A file is an MP4 container that holds only audio. The same container specification as a video MP4 — the identical box, with the video track left out.
Apple introduced the extension so that systems would not try to open an audio-only file in a video player. That is the entire purpose of the name: it signals intent. The container and the codec are separate layers, and M4A describes only the first.
Inside an M4A you will find one of two very different codecs:
AAC — lossy, and by far the more common. It discards detail to achieve small files, the successor to MP3 and better than it at the same bitrate. A three-minute song is roughly 3 to 6 MB.
ALAC (Apple Lossless) — lossless. It compresses without discarding anything; decode it and you get the original samples back exactly. The same three-minute song is roughly 20 to 30 MB.
So a five-times to ten-times size difference between two M4A files is not an error, and neither file is corrupt. One is a compressed approximation and the other is a perfect copy. The extension tells you nothing about which you have.
The quickest way to tell: look at the size against the duration. Anything near 10 MB per minute is ALAC; anything near 1 MB per minute is AAC. A tool that reports audio file information will name the codec outright.
Several extensions you may have met are the same container with a different label, used to tell Apple software how to treat the file:
.m4a — ordinary audio..m4b — audiobooks. Identical container; the extension makes players remember your position and offer chapter navigation and speed control..m4r — ringtones..m4p — the old DRM-protected purchases. These are genuinely locked, not merely labelled.Because .m4a and .m4b are the same format, renaming a long audio file to .m4b is a real trick: it turns a recording into something a player will treat as an audiobook, with position memory. Nothing inside the file changes.
That third case is worth dwelling on. Pulling the audio out of an MP4 into an M4A does not need re-encoding at all — the track moves from one box to another untouched, in seconds, with no quality loss. If a tool takes several minutes to extract audio from a video and the result sounds slightly worse, it re-encoded something it did not need to.
AAC in an M4A plays on essentially every phone, computer, browser and modern car system. The exceptions are older hardware — car stereos and standalone players from the MP3 era — which may show the file and refuse it, or not list it at all.
ALAC is narrower. It is well supported across Apple software and increasingly elsewhere, but a device that happily plays AAC may not handle ALAC, and the failure looks identical from the outside.
The right answer depends entirely on which codec you actually have.
From AAC: this is lossy to lossy. The file is decoded and compressed again, and the second pass can only remove more. The result is never better than what you started with. Do it only for compatibility, and use a generous bitrate — 192 kbps or higher — so the second round of damage stays inaudible.
From ALAC: this is the normal, healthy direction. You are compressing a lossless original for the first time, exactly as you would from a WAV, and the result is as good as MP3 gets. Keep the ALAC as your master.
So the same operation — “convert M4A to MP3” — is a reluctant compromise in one case and the correct workflow in the other, and the file extension gives you no clue which situation you are in.
.m4b, .m4r and .m4p are the same container with different labels