Format
MKV, a container that holds anything.
Matroska can carry any video, many sound tracks, subtitles and chapters in one file. That freedom is also why players disagree about it.
Flex Convert reads MKV and writes MP4. The sound is the part that can go wrong.
Facts
The record.
Extension
.mkv for video, .mka for sound only. WebM is a restricted form of the same container.
What it is
Matroska, an open and free container format started in 2002.
What it holds
Any video codec, several sound tracks in different formats, subtitles, chapters and attached files.
Who writes it
OBS and other recorders, video downloaders and disc rippers.
Who reads it
VLC, Plex, Jellyfin, Kodi and most Android players. QuickTime, Photos and many TVs and editors do not.
In Flex Convert
Read when the browser can decode the tracks. The one video track is kept, and the sound tracks the browser can read.
Measured examples
What we measured.
Four generated 10-second 720p clips with the same H.264 picture and four different sound tracks, converted to MP4.
| Test file | Size | Change | Time | Result |
|---|---|---|---|---|
| 10 s clip, AAC soundChrome | 1.9 MB → 1.9 MB | Same size | 180 ms | Video stream identical · Copied, no quality lost |
| 10 s clip, AC-3 soundChrome | 2 MB → 1.7 MB | 12 percent smaller | 190 ms | Video stream identical · Sound left out · The sound couldn't be converted and was left out |
| 10 s clip, Opus soundChrome | 1.9 MB → 1.9 MB | 3 percent smaller | 410 ms | Video stream identical · Sound converted |
| 10 s clip, Vorbis soundChrome | 1.8 MB → 1.9 MB | 4 percent larger | 400 ms | Video stream identical · Sound converted |
Measured on November 7, 2026 in Google Chrome 157, on Apple M1 Pro. Times run from pressing Convert to the result being ready, with the page already loaded. Likeness is PSNR against the original: higher is closer, and above 40 dB the difference is hard to see. Every test file was generated for these measurements, none is anyone else's work, and each is described on the About page.
The video was copied in all four. AAC sound was copied in 181 milliseconds. Opus and Vorbis sound became AAC. The AC-3 clip came back with no sound at all, and the page said so.
Good to know
Why MKV is flexible and fussy.
Freedom in the container, rules at the player.
The sound problem
MKV files from films and rips often carry Dolby Digital (AC-3), Dolby Digital Plus or DTS. Browsers do not decode them and we do not ship a decoder, because those codecs are patent-encumbered. The picture converts and the file comes back without sound, with a note on the file row.
If the MKV has an AAC track as well, that track is kept.
What does not come across
An MP4 has no equivalent for most of what Matroska carries. Subtitle tracks are not copied, chapters are dropped, and only the first readable sound track is used.
Recordings from OBS
OBS can record to MKV so a crash does not lose the file. When the recording is finished, converting to MP4 gives an editor-friendly copy. With H.264 and AAC that is a copy taking seconds, even for hours of footage.
Choosing
When to pick it, and when not to.
A format is a trade. These are the trades this one makes.
Keep MKV when
you play it in VLC, Plex, Jellyfin or Kodi. They play MKV directly and a conversion only costs time.
Convert to MP4 when
the destination is fussy: QuickTime, Photos, a phone gallery, a web form or a TV with a short list of formats.
Use a desktop tool when
the file has Dolby Digital or DTS sound you want to keep, or subtitles you need.