MKV → MP3
MKV to MP3 converter
The first audio track in the Matroska file is decoded, mixed down to stereo if it is surround, and encoded to MP3 with LAME. The video is skipped, and the MP3 is lossy even when the MKV track was lossless.
…or drag files and folders here. Add as many as you like — everything is processed on this device.
.avi .f4v .m4v .mk3d .mka .mkv .mov .mp4 .qt .weba .webm
In one paragraph
MKV is the container of choice for film and TV rips, OBS recordings and anime releases, largely because it can hold almost any codec: AAC, AC-3 (Dolby Digital), E-AC-3, DTS, Opus, Vorbis and FLAC are all common. That flexibility is also why so many players and phones refuse MKV files, and why getting the soundtrack, commentary or a recorded stream out as a plain MP3 is a frequent need.
Sonic Batch reads the MKV in a WebAssembly build of FFmpeg inside your browser, so the codecs browsers lack, AC-3 and DTS especially, still decode. A 5.1 or 7.1 track is mixed down to stereo with FFmpeg’s standard layout-aware downmix, because MP3 holds at most two channels, and then encoded with LAME at the bitrate you set.
Does this change the audio quality?
MKV soundtracks are often AC-3 at 384–640 kbps or even lossless FLAC, so a 192–320 kbps MP3 is a real reduction. Mixing 5.1 to stereo also changes the sound: dialogue from the center channel and effects from the rear are folded into two speakers. For a faithful copy, use MKV to WAV.
- Only the first audio track is extracted. MKV files with several languages or a commentary track will give you the default one, usually the first listed.
- Surround audio is mixed down to stereo. The MP3 cannot hold 5.1.
- Subtitles, chapters and attachments in the MKV are ignored.
- Files over 400 MB are rejected before decoding, which rules out most full-length films.
A worked example
Saving a recorded live stream as an MP3 for listening later
- Drop the OBS MKV recording into the queue.
- Choose the Podcast/voice preset if it is mostly talking, or keep Balanced (192 kbps stereo) for music.
- Turn on Trim to skip the starting-soon screen at the beginning.
- Convert and download the MP3.
Go to 256 kbps for film soundtracks, 96 kbps mono for talk
A downmixed movie or concert soundtrack has wide dynamics and dense effects, so 256 kbps keeps it clean. A recorded stream, podcast or lecture in MKV is speech, and 96 kbps mono is indistinguishable from the source while being a quarter of the size. Avoid 320 kbps unless the MKV track was lossless; it rarely helps.
Supported formats, limits and browser notes
Sonic Batch decodes with the browser when it can (faster, no engine download) and falls back to the self-hosted WebAssembly engine when it cannot. Every combination below works in every listed browser; the column only tells you which decoder does the work.
| Input | What it is | Chrome | Firefox | Safari | iOS |
|---|---|---|---|---|---|
| MKV | Open video container used for films, TV recordings and OBS captures. Audio is often AAC, AC-3, DTS, Opus or FLAC. | Partial | Partial | Engine | Engine |
| WebM | Open container used by browser recorders and YouTube downloads. Audio is Opus or Vorbis. | Native | Native | Partial | Partial |
| MP4 | The standard video container. Its audio track is almost always AAC. | Native | Partial | Native | Native |
| MOV | QuickTime video from iPhone and DSLR cameras. Audio is normally AAC or PCM. | Partial | Engine | Native | Native |
| AVI | Legacy Windows video container from camcorders, older cameras and DivX/Xvid files. Audio is usually MP3, PCM or AC-3. | Engine | Engine | Engine | Engine |
| Output | Encoder | Bitrates / depth | Sample rates | Tags |
|---|---|---|---|---|
| MP3 | libmp3lame | 32–320 kbps | 8–48 kHz | Yes + artwork |
- Max per file
- 400.0 MB
- Max per queue
- 4.00 GB
- Max duration
- 4:00:00
- Max archive
- 1.50 GB
These are enforced ceilings, checked before any decoding starts, not measured maximums — a file over the limit is rejected rather than allowed to exhaust the tab’s memory. Real-world capacity is lower on older phones, which is why the scheduler runs one file at a time on iOS and Safari.
Where your audio goes
Nowhere. Your file is read from disk into this browser tab with the File API, decoded and encoded by code running in that same tab, and handed back to you as a download. It is never transmitted.
That is a structural property of the application, not a policy promise: there are no API routes, no server actions and no third-party processing services in this project, so there is nothing capable of receiving a file. The page’s Content-Security-Policy also restricts connections to this origin, and the automated test suite includes a browser test that fails if any network request during import, preview, conversion or download carries media bytes.
The trade-off, stated plainly: results live in your tab’s memory until you download them. Closing or reloading the page discards them. There is nothing on a server to recover, because nothing was ever put there.
Troubleshooting
I got the wrong language track.
This page always extracts the first audio track in the file. If you need another one, remux the MKV with a desktop tool such as MKVToolNix to put that track first, then convert here.
Dialogue is quiet compared to the music and effects.
That is common when a 5.1 film mix is folded to stereo, because the center channel is mixed in at a lower level. Turn on loudness normalization in the batch converter, or raise the volume in your player.
The file loads but conversion fails.
Some MKV files use a codec FFmpeg cannot decode in the browser build, or are damaged at the end from an interrupted recording. The row shows the codec the engine found; DTS-HD MA and TrueHD tracks decode, but very large ones may exceed the memory budget.
Frequently asked questions
How do I convert MKV to MP3?
Add your MKV files above, choose a bitrate, and press Convert all. Download the MP3s one by one or as a ZIP.
Can it read Dolby Digital (AC-3) and DTS audio?
Yes. The engine includes decoders for AC-3, E-AC-3, DTS and TrueHD, which browsers do not have. The track is mixed down to stereo for the MP3.
Is the MKV uploaded?
No. Everything runs in your browser tab with a self-hosted engine. There is no server that receives files.
Can I convert MKV to MP4 here?
No. This is an audio tool and never writes video. It only extracts sound from video files.