Do the audio tracks inside an MKV make the file bigger?

Yes, every additional audio track adds its own data to the file and increases the total size. But audio data stays quite small next to picture data; a compressed audio track is a very small fraction of what the video track carries over the same duration. Adding a second language to a film therefore does not double the file, it only enlarges it somewhat. Subtitle tracks, being text, are practically negligible in size terms.

Can MP4 really not carry multiple audio tracks?

Technically the MP4 standard can define more than one audio track, so it is not entirely impossible. The problem is not in the standard but in implementation; the number of players that properly show secondary audio tracks inside an MP4 and let you switch between them is limited. The situation is even more restricted on the subtitle side, because MP4's subtitle support recognises fewer formats than MKV's. That is why MKV is preferred in practice for multi-track files.

What is the difference between a subtitle track and burned-in subtitles?

A subtitle track is text that sits inside the file as a separate layer and is drawn on screen by the player when requested; you can turn it on and off, and even change the typeface. Burned-in subtitles, by contrast, are written permanently into the pixels of the picture and there is no way to switch them off. Burning in requires the video to be re-encoded and therefore causes some quality loss. What MKV offers is the first kind, that is, flexible subtitles you can switch off.

If a file has both Turkish and English audio, which one does the player choose?

That depends on the flags on the tracks and on the player's own settings. MKV can attach a language tag to each track along with whether it is marked as default; the player then usually picks the track flagged as default or the one matching the system language. If no language tag has been written to the tracks, the player will mostly open the first one in the list. This is why adding correct language tags to your tracks pays off considerably in the long run when you are archiving.

How the MKV Container's Multi-Audio and Subtitle Advantage Works

One file, several worlds

Anyone who keeps a film archive has ended up in this situation: the Turkish-dubbed version of a film sits in one file, its original English audio in another, and the subtitles in a third .srt file. The folder falls apart, it becomes unclear which subtitle matches which version, and when you move one file you forget the other.

MKV is a container designed to make that problem disappear. A single .mkv file can carry as many audio languages, as many subtitles and as many chapter markers as you like alongside the same picture. So how is this possible, and why can MP4 not comfortably do the same thing?

What a container does and does not do

Let us clear up the basic distinction first, because the confusion around this subject always comes from the same place.

MKV, MP4, MOV, WEBM — these are all containers. A container does not compress the picture, does not determine quality, does not shrink the file. Its only job is to hold the pieces placed inside it together in an orderly way and to guide the player: "this is here, that begins over there".

The thing that compresses the picture is the codec: H.264, H.265, AV1 and so on. Audio is compressed by separate codecs: AAC, MP3, Opus, or uncompressed PCM.

The importance of that distinction here is this: MKV's advantage is not quality, it is organisation. An H.264 picture inside an MKV and the same H.264 picture inside an MP4 are bit-for-bit identical. What makes MKV valuable is not that it shows the picture better, but that it can hold more things next to it.

Track logic: parallel lanes inside the file

Think of the inside of an MKV file as lanes running side by side. Each lane is a track, and each track carries its own type, its own codec, its own timestamps.

In a typical archive file you will see these tracks:

Video track: usually just one, compressed with H.264 or H.265
Audio track 1: Turkish dub, AAC
Audio track 2: original English, AAC or a multichannel format
Subtitle track 1: Turkish
Subtitle track 2: English
Subtitle track 3: Turkish with descriptions for the hard of hearing
Chapter list: time markers showing where scenes begin

When the player opens the file it reads the list of these tracks and presents you with a menu. When you switch the audio from Turkish to English the file is not reopened and no new file is loaded — the player simply changes which lane it decodes. It happens instantly because both tracks are already there.

Labels can also be added to each track: a language code, a track name (such as "Director's Commentary"), whether it is default, whether it is a forced subtitle. When these labels are filled in properly the player picks the right track by itself and you start watching in the right language without doing anything.

MKV's second hidden strength: chapters

Although it is not discussed as much as multiple audio and subtitles, MKV's chapter support is a serious advantage for archivists. Chapter markers are named points in time within the file: "Intro", "First Scene", "Finale". The player displays them as a menu and you jump to that point with a single press.

In long content — a conference recording, a lecture series, a documentary — this feature makes the file navigable. You are not forced to take blind shots at the progress bar to find the section you are after.

So why doesn't MP4 do the same?

The MP4 standard can also technically define more than one audio track. The issue is not "MP4 cannot do this" but "this does not work well in the MP4 ecosystem".

The reason lies in MP4's design philosophy. MP4 spread as a distribution format: a video would be uploaded, sent to everyone, and open on every device on the first attempt. That goal required a narrow and precise standard — specific codecs, a simple structure, no surprises. That constraint made MP4 universal but took away its flexibility.

MKV was born with exactly the opposite philosophy: an open, extensible container able to carry nearly anything. On the subtitle side the difference becomes especially pronounced; MKV can hold a variety of subtitle formats as tracks, while MP4's support in this area is far narrower.

The upshot is that when you want to turn an MP4 in your hands into a multi-track archive file, MKV is the right target. What is more, because converting MP4 to MKV does not require re-encoding the picture, you suffer no quality loss — the data is carried over into the new container as it is.

What it costs: compatibility

To be honest, MKV's flexibility has a price, and that price is not small.

The built-in player in many smart televisions does not recognise MKV, or recognises it only with certain codecs. The default video app on iPhone and iPad struggles with MKV. Some video editing programs will not import MKV files directly. You cannot upload MKV to social media platforms or to most web upload forms.

The practical approach is therefore this: let your archive copy be MKV and your sharing copy MP4. Keep the full version with all its tracks, subtitles and chapters as MKV; keep the version you will send to someone or play on a television as an MP4 with a single audio track.

Adding tracks later: MKV's quiet superiority

The least discussed but, for archivists, most valuable aspect of multi-track support is this: you can add a track to an MKV file after the fact, and that operation never touches the picture.

Say you have archived a documentary. Months later a better-translated Turkish subtitle comes your way. All you have to do with MKV is add that subtitle to the file as a new track. The video track stays as it is, is not re-encoded, and not a single frame is degraded. The operation finishes very quickly compared with recompressing the whole file.

The same flexibility works in the other direction. If a file holds three separate languages you do not need, you can shrink it by removing the unnecessary tracks — again without touching the picture. This is a completely different and far safer method than shrinking a file with lossy compression.

The general principle here coincides with the basic rule of archive management: the less you touch the picture data, the better. Every re-encode adds an irreversible loss. MKV's track logic lets you keep updating a file over the years while never putting the video track at risk. Doing the same things with MP4 is often impossible or requires producing the file again from scratch.

In summary

MKV's multi-audio and subtitle advantage is not a matter of quality but a matter of order. The container holds each piece inside it as a separate track and can attach a language tag to every track; the player then switches between those tracks instantly. Scattered piles of files and subtitles are gathered into a single file, and chapter markers make it navigable as well. What you pay in return is device support. If you want to keep a file for years, the order MKV offers is well worth that price; if you are going to send the file to someone else, MP4 is still the safer harbour.