Reverse M4A

The letters M4A name a box, not a codec. Apple’s container can hold AAC (lossy) or ALAC (Apple Lossless). iPhone Voice Memos, GarageBand shares, iTunes rips, and some Android exports all show up as .m4a. This page reverses the audio inside that box in your browser. You upload the file, the decoder produces PCM, the timeline runs backwards, you preview, you download WAV. The original M4A never goes to a server.

If your whole question is “how do I do this on the phone,” start with how to reverse audio on iPhone and reverse voice memo. If you wanted a native App Store binary, reverse audio app iOS explains that this origin is still a website. Lossy cousins: MP3 reverser. PCM cousins: reverse WAV online.

AAC versus ALAC in the same extension

AAC is a lossy codec, in the same family as the audio inside many MP4 videos. Voice Memos uses it because memos would be huge as PCM. You get small files and the usual lossy trade: dull air, pre-echo on claps, swish on “s” sounds. Reverse does not undo that. You hear the damage in reverse order.

ALAC is lossless inside the same .m4a wrapper. CD rips from iTunes, some Bandcamp files, and “Apple Lossless” exports use it. Decode should match the original PCM. Reverse then behaves like a WAV reverse, with the same browser-memory limits as other lossless files.

You cannot trust the icon. Open Get Info / file details if you must know. For this tool it rarely matters: both decode to a buffer, both reverse, both download as WAV. It matters when you judge quality. An AAC memo will never “become” ALAC because you reversed it.

Voice Memos workflow

1. In Voice Memos, open the take.
2. Share or Save to Files. You want the M4A, not a video.
3. On this page, upload that M4A. Other accepted types are MP3, WAV, FLAC, OGG, and AAC as .aac.
4. Wait for decode. Long memos allocate a large buffer on the phone; if Safari reloads, split the memo or use a desktop.
5. Play. Switch between Play reversed and Use original. Prank memos that were already reversed should sound like speech in one of the two modes.
6. Preview 0.5x–2x to pick words. Speed is preview only.
7. Download WAV. Import into Files, iMovie, or a DAW.

Microphone capture on this page maxes out at 60 seconds. A Voice Memo you already recorded can be longer; upload beats re-recording.

Processing stays on the device. That is the point for memos that contain private speech.

iTunes and music M4A

Store-bought tracks may be AAC. Some are still wrapped with FairPlay. If the browser cannot decode, the file is protected or broken; this site will not strip DRM.

For a track you exported yourself from a session as M4A, reverse is fine for a listen. For a release, bounce WAV or ALAC from the DAW and reverse that bounce.

This page does not accept a YouTube or Apple Music URL. Save a file you are allowed to use, then upload.

Container atoms that disappear

M4A can store cover art, lyrics, chapter markers, and gapless flags. The reverse path ignores them. The WAV out is samples plus a standard header. If you needed chapters (a podcast M4A), reverse is the wrong first step; split chapters in an editor, reverse each region if you must, and rebuild.

Why the export is WAV, not M4A

Writing M4A would mean choosing AAC or ALAC and running an encoder in the browser. AAC-out would stack loss on Voice Memos. ALAC-out is possible in principle but is another codec to support and test across Safari versions. WAV plays in iOS Files, most editors, and song ID apps. If Messages or a client insists on M4A, encode on the phone after you approve the reverse.

What the engine will not do

No polarity invert. No speaker identification. No translation. No song database. No batch folder of an iTunes library. One file at a time.

Stereo memos (some Bluetooth mics) reverse per channel. Mono memos stay mono.

Failures you will see on iPhone

• “Could not decode”: try another browser, or share the memo as WAV from a desktop Voice Memos sync.
• Tab crash: file too long for RAM. Split the memo.
• Download missing: check the Files download list; iOS sometimes hides it.
• Chat app rejects WAV: convert locally.

Safari’s decoder is usually enough for Voice Memos AAC. If you recorded in a third-party app that writes unusual profiles (HE-AAC, xHE-AAC), decode can fail. Bounce to WAV in that app if so.

After reverse

Two passes restore word order, with a short edge fade so the round trip is not bit-identical. Keep the original M4A.

If the goal was a reversed ad-lib in a beat, drop the WAV on a timeline and trim. If the goal was understanding a prank, transcribe after you hear Normal-quality speech in one toggle position.

M4A is how Apple ships everyday audio. This page exists so you do not need a DAW for the reverse step on those files. For the wider iPhone how-to, use the linked guides; for the codec math, remember: container is M4A, codec is AAC or ALAC, result here is WAV.