Extract the audio from an MP4 as M4A
This is not really a conversion — it is an extraction. The video track is thrown away and the audio is written out as a standalone M4A. The file gets dramatically smaller, because the picture was almost all of it.
What is actually happening
An MP4 is a container holding (at least) two streams: video and audio. They sit side by side, independently encoded, synchronised by timestamps.
Extracting to M4A means discarding the video stream entirely and writing the audio out on its own. Convexy exports the audio track through Apple's AAC exporter into an M4A container.
The convenient part is that the audio inside an MP4 is already AAC, and M4A is the container AAC belongs in. You are landing the audio where it already wanted to live. The honest caveat is that this is an export, not a guaranteed bit-for-bit copy of the original audio track — treat it as one more encode rather than a free remux. In practice, from any normally-encoded video, that is not something you are going to hear.
Expect the file to collapse in size
In almost any video, the picture is the overwhelming majority of the bytes. Audio typically runs at 128–256 kbps; 1080p video runs at several thousand.
So a 500 MB one-hour recording of a lecture, a webinar or a conference talk will commonly come out somewhere in the region of 50–100 MB as audio alone — sometimes far less. You are not compressing anything. You are simply no longer storing the picture.
When you want this
- Lectures, talks and webinars you want to listen to on a walk rather than watch at a desk.
- Interviews and meetings where the audio is the record and the video is just a camera pointed at a room.
- Podcast source material recorded to video and needing to become audio.
- Transcription. Most transcription tools take audio, and handing them a 500 MB video when a 50 MB M4A will do is a waste of everyone's time.
- Music, live sets or performances captured to video that you would rather have as a listenable file.
Choosing the output format. M4A is the right default — it is what the audio already is. Convexy can also extract to MP3 (when something old demands MP3), or to WAV and FLAC. Be clear-eyed about those last two: the audio in the video was lossy AAC, so wrapping it in a lossless format gives you a much larger file containing exactly the same audio. It does not recover anything. Choose WAV or FLAC only because a tool insists on them, never for quality.
What Convexy writes
Convexy exports the M4A using Apple's own AAC exporter at its standard high-quality setting, on your device. There is no bitrate dial for M4A output — Apple's exporter does not offer one. If you need a specific bitrate, extract to MP3, where 128, 192, 256 and 320 kbps are all selectable, along with sample rate and channels.
For a lecture or an interview, extracting to MP3 at 128 kbps in mono is a legitimate choice: it produces a very small file, and a single voice in a room was never stereo in any meaningful sense.
Why doing it on the device matters here
Look at that list of use cases again. Interviews. Meetings. Lectures. Medical or legal recordings. Family video. These are precisely the files you would least like to hand to a stranger's server so it can perform a thirty-second operation and promise to delete them afterwards.
Convexy has no server. The extraction runs on your iPhone or iPad, works in Airplane Mode, and requires no account. Nothing is uploaded, because there is nowhere for it to go.
How to do it
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Bring the MP4 into Convexy
Tap Browse files to pick it from Files, choose it from Photos, or share the video into Convexy from another app.
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Choose M4A
Convexy lists what the file can actually become — M4A sits alongside MOV, M4V, animated GIF and still frames.
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Convert
The video track is discarded and the audio is exported on your device. Large files take a moment; nothing is uploaded.
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Rename it if you need to
Convexy lets you rename the output before saving — useful when the video had a camera-generated filename.
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Save or share it
Save the M4A to Files, or send it straight to a transcription tool, a podcast editor, or whoever needs it.
Common questions
Does extracting audio from an MP4 lose quality?
Barely, but be precise about it: the audio in the video is already lossy AAC, and Convexy exports it through Apple's AAC exporter rather than copying the bits verbatim. Treat it as one more encode, not a free remux. From any normally-encoded video it is not something you will hear — and it certainly does not degrade with the video track's removal.
Should I extract to M4A, MP3, WAV or FLAC?
M4A unless you have a reason. It is the container the audio is already in, so it is the most direct landing. Choose MP3 if the destination is old hardware or a form that insists on it. Choose WAV or FLAC only if a tool demands them — the audio was lossy AAC, so a lossless container just gives you a much bigger file containing exactly the same audio.
Why is the M4A so much smaller than the MP4?
Because the picture was nearly all of it. Video runs at thousands of kilobits per second; audio at a couple of hundred. A one-hour 500 MB recording routinely becomes 50 MB or less as audio alone. You have not compressed anything — you have just stopped storing the images.
Can I set the bitrate of the extracted M4A?
No. Convexy exports M4A with Apple's AAC exporter at its standard setting, and that exporter does not expose a bitrate control. Extract to MP3 if you need to choose a specific bitrate — 128, 192, 256 and 320 kbps are all available there, along with mono, which is a large saving on speech.
Is the video file changed?
No. Convexy writes a new audio file and leaves the original MP4 exactly as it was. Nothing is modified, moved or deleted.
Does this upload my video?
No. There is no server. The extraction happens on your iPhone or iPad and works with the network turned off — which is exactly what you want for a recorded meeting, interview or lecture.