Convert MKV to M4A
Pull the audio out of an MKV as an M4A. What happens next depends on what is inside, and it is worth knowing which case you are in: an MKV holding AAC — which most web-sourced files do — has its track copied out untouched, bit for bit. One holding AC-3 or DTS, which most films do, is decoded and re-encoded, and its surround channels are folded to stereo.
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Up to 50 MB. Deleted automatically after 30 minutes.
What it does
- An AAC track is copied, not re-encoded
- AC-3 and DTS are decoded and mixed to stereo
- The video is discarded, so it finishes fast
- No watermark and no sign-up
How to use MKV to M4A
- 1
Trim first if the file is long
MKV holds films and episodes. Cut the section you need before uploading — the limit here is 50 MB.
- 2
Add the MKV
Drop the trimmed file in or choose it from your computer.
- 3
Convert
The video is discarded. An AAC track is copied into an M4A container; anything else is decoded and re-encoded to AAC.
Two conversions wearing one name
Matroska was built to hold anything, which is why this page has to describe two different operations. What you get depends entirely on what somebody put inside the file.
An MKV holding AAC — which is what screen recorders, stream downloaders and most web-sourced video produce — needs nothing done to it. AAC is what an M4A holds, so the track is lifted from one container into the other untouched. The result is bit-for-bit the audio that was recorded.
An MKV holding AC-3 or DTS — which is what films, disc rips and anything with surround sound carry — cannot be copied, because an MP4 container has no way to describe those codecs. That audio is decoded and re-encoded to AAC, and its five or six channels are folded into two.
How it works
The video stream is discarded rather than converted. The source's audio codec is
then read: if it is already AAC the track is copied with -c:a copy, and if it
is anything else it is decoded and encoded to AAC.
Nothing about that is a setting you choose. It is decided by the file, and both outcomes are the right answer for their case.
What a downmix costs, and what it does not
Folding six channels into two is not a loss of fidelity. Every sound in the mix is still in the output; they are simply sharing two channels instead of six.
What goes is position. For a film that means dialog sits less clearly in front of the music, because a center channel exists mainly to keep those apart. For music recorded in surround, instruments separated by where they were become separated only by level and frequency.
If placement matters for what you are doing, the MKV is the only copy that has it, and a desktop tool can extract the multichannel track without mixing it.
Picking a specific track
An MKV with several audio tracks gives you the first one here, which is usually but not always the one you want — a commentary or a second language is often first in files assembled by hand.
MKVToolNix lists what is inside a file in seconds, and ffmpeg can extract a chosen track by index, frequently without re-encoding it at all. That is the better route whenever the choice matters.
Examples
A recorded talk saved as MKV
Screen and stream recorders write MKV with AAC audio, so the track comes out exactly as it went in.
A film soundtrack in surround
Five or six channels of AC-3 folded into two. Everything is still there; what is lost is where each sound was placed.
Frequently asked questions
How do I know whether mine was copied or re-encoded?
By what the MKV holds. Recordings from OBS, screen capture and downloads generally carry AAC, and those are copied — the result is bit-for-bit the audio that was recorded. Films and disc rips generally carry AC-3 or DTS, which an MP4 container cannot hold, so those are decoded and re-encoded to AAC.
What happens to surround sound?
It is mixed down to two channels, because M4A here is stereo. The content is all still present; what is lost is placement. A sound deliberately positioned behind the listener arrives in the middle with everything else, and dialog from a center channel sits alongside the music rather than in front of it.
Which track is taken if there are several?
The first one in the file. MKV commonly holds more than one — a second language, a commentary, a stereo mix alongside a surround one — and there is no way to choose here. A desktop tool such as MKVToolNix or ffmpeg will list them and extract a specific one, often without re-encoding.
Should I choose M4A or MP3?
M4A when you know what will play it — an iPhone, a Mac, podcast software, a recent car — because it is lossless for an AAC source and better than MP3 at the same size. MP3 when the destination is old or unknown, because nothing refuses MP3.