Convert MKV to MP3
Pull the audio out of MKV files as MP3 — concerts, lectures, rips and recordings. Batch queue, bitrate control, no size cap, nothing uploaded.
- Runs in your browser
- Files are never uploaded
- Free, no account
First use downloads the 10 MB video engine from this site. After that it is cached and works offline. It is WebAssembly served from onthisdevice.com — no third-party CDN — and your file is still processed entirely on your own device.
Why MKV audio extraction is worth doing locally
MKV files are the large ones: concert recordings, conference talks, archived broadcasts, Blu-ray rips. They are exactly the files upload converters refuse — a free tier that caps at 500 MB is useless against a 6 GB Matroska file, and even where it is allowed, uploading six gigabytes to extract 150 MB of audio is an absurd use of a connection.
Here the video is simply never decoded. FFmpeg reads the audio stream out of the container, re-encodes it with LAME and writes an MP3. The video track — usually more than 95% of the file — is discarded without ever being touched, which is why extraction is so much faster than a conversion.
What FFmpeg finds inside a typical MKV
| Audio codec in the MKV | Where it comes from | Converts to MP3? |
|---|---|---|
| AAC | Most re-encodes and web sources | Yes |
| AC-3 / E-AC-3 | Broadcast and DVD sources | Yes, downmixed to stereo |
| DTS | Blu-ray sources | Yes, downmixed to stereo |
| FLAC | Lossless music rips | Yes — use 320 kbps, the source is lossless |
| Opus / Vorbis | WebM-derived and open-source encodes | Yes |
| PCM | Capture cards and recorders | Yes |
Multiple audio tracks
Matroska is the container people choose precisely because it can hold six audio tracks and a commentary. This page takes the first stream, which is the default track in virtually every file — the original language in a film, the room mic in a recording. Choosing track four means stream mapping, which is a genuinely fiddly command-line operation, and it is deliberately not exposed here rather than being half-implemented behind a confusing dropdown. For that specific job, a desktop FFmpeg or MKVToolNix is the right tool.
Surround sound becomes stereo
MP3 is a stereo format for practical purposes, so a 5.1 AC-3 or DTS track is downmixed to two channels on the way through. Dialogue from the centre channel is folded into both sides, which is what you want for headphones and phone speakers. If you need the discrete channels preserved, MP3 is the wrong destination format entirely.
How much smaller is the result?
A two-hour 4 GB film with a 192 kbps MP3 soundtrack comes out around 165 MB — roughly one twenty-fifth of the original. The arithmetic is simply bitrate × duration: 192 kbps × 7,200 seconds ÷ 8 bits ÷ 1,024 ÷ 1,024. The page shows the before and after sizes for every file it converts.
Doing it offline
Once the FFmpeg engine has been fetched from this site the first time, it stays cached. Put the browser into offline mode, or actually disconnect, and every conversion on this page still works — which is the simplest available proof that your file is not going anywhere.
Questions
My MKV has several audio tracks — which one is used?
The first audio stream, which is the default track in nearly every file. Selecting a specific language track needs stream mapping that this page deliberately keeps out of the way; if you need track three, use a desktop tool for that one job.
The MKV holds FLAC or DTS audio. Will that work?
Yes. FFmpeg decodes FLAC, DTS, AC-3, Opus, Vorbis and AAC alike, then re-encodes to MP3. Converting lossless FLAC to MP3 does lose data by definition, so pick 320 kbps if the source was lossless and the result matters.
Why is the MP3 so much smaller than the MKV?
Because the video — which is typically 95% of the file — is thrown away entirely. A 4 GB film with a two-hour soundtrack becomes roughly 170 MB at 192 kbps.
Is anything uploaded while it works?
No. The FFmpeg build that does the decoding is WebAssembly served by this site and runs inside the tab. You can put the browser in offline mode after the engine has loaded once and every conversion still works.