MP4 → MP3
MP4 to MP3 converter
The audio track inside the MP4 is decoded and re-encoded to MP3 with LAME; the video is skipped entirely. MP4 audio is usually AAC, so this is a lossy-to-lossy step, and 192 kbps is the sensible ceiling.
…or drag files and folders here. Add as many as you like — everything is processed on this device.
.3g2 .3gp .asf .avi .f4v .flv .m4v .mk3d .mka .mkv .mov .mp4 .qt .weba .webm .wmv
In one paragraph
MP4 is the default video format of almost everything: Android phones, GoPros, Zoom and Teams recordings, screen recorders, and most video you have ever downloaded. When what you actually need is the sound (a lecture to listen to on a walk, a webinar for your podcast feed, a recording of a rehearsal), carrying the whole video around is wasteful. A 10-minute 1080p clip is often 200 MB or more; its audio as a 192 kbps MP3 is about 14 MB.
This page reads the MP4 in a WebAssembly build of FFmpeg that runs inside your browser tab, picks the first audio track, ignores the video stream, and encodes MP3 with LAME. Nothing is sent anywhere. That matters more for video than for any other file type: video is large, so an upload-based converter makes you wait for the upload before any work starts, and it is often personal footage you would rather not hand to a stranger’s server.
Does this change the audio quality?
Phone and camera MP4s almost always carry AAC at 96–256 kbps, so the MP3 is a second lossy generation and cannot sound better than the AAC it came from. At 192 kbps the extra loss is very hard to hear; going above the source bitrate only makes the file bigger.
- Only the first audio track is extracted. A second commentary or language track is left out.
- Surround (5.1) audio is mixed down to stereo, because MP3 holds at most two channels.
- Files over the 400 MB per-file budget are rejected before decoding. Trim long recordings in your video app first.
- Protected (DRM) video from streaming services cannot be read and is marked unsupported.
A worked example
Turning a semester of recorded lectures into a podcast-style playlist
- Drop the folder of lecture MP4s onto the drop zone. Each row shows its duration once the file has been read.
- Choose the Podcast/voice preset: mono, −16 LUFS loudness, and a bitrate that keeps an hour of speech around 30 MB.
- Press Convert all. Only the audio is processed, so each lecture takes a fraction of its running time.
- Download the ZIP and import the MP3s into your phone’s podcast or music app.
Pick the bitrate from the content, not the video quality
A 4K video does not have better audio than a 720p one; phones record the same AAC track either way. For music, concerts and anything you will edit, use 192 kbps stereo. For talks, lectures and meetings, mono at 96 kbps sounds identical to the source and is half the size. Only choose 256–320 kbps when the MP4 came from a camera or recorder that captured uncompressed PCM audio.
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 |
|---|---|---|---|---|---|
| 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 |
| 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 |
| 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 |
| WMV | Windows Media video in an ASF container. Audio is almost always WMA (Windows Media Audio). | Engine | Engine | Engine | Engine |
| FLV | Flash-era web video, still produced by some stream recorders. Audio is AAC or MP3. | Engine | Engine | Engine | Engine |
| 3GP | Legacy phone recording container, normally AMR-NB or low-bitrate AAC audio. | Engine | Engine | Partial | Partial |
| 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
It says the file has no audio stream.
Some screen recordings and muted exports really contain no audio track. Play the MP4 in a video player first: if it is silent there, there is nothing to extract.
The MP3 is shorter or longer than the video by a fraction of a second.
Video containers often start the audio a few milliseconds before or after the first frame, and MP3 adds a short encoder delay. The audio content is complete; only the padding differs.
My MP4 from a streaming service will not convert.
Purchased or rented video from streaming platforms is usually encrypted with DRM. The engine cannot decrypt it, and the file is marked unsupported rather than producing silence.
Conversion is slow on my phone.
Mobile Safari runs the single-threaded engine and one file at a time, and it still has to read through the whole video file to find the audio. Long clips can take a minute or two; desktop browsers are much faster.
Frequently asked questions
How do I convert MP4 to MP3?
Choose or drop your MP4 files above, keep the default 192 kbps setting or pick a preset, and press Convert all. Download each MP3 as it finishes, or all of them as a ZIP. Everything happens in this browser tab.
Is my video uploaded?
No. The FFmpeg engine runs inside your browser, the page has no upload endpoint, and the Content-Security-Policy blocks connections to any other site. Your video never leaves the device.
Does converting MP4 to MP3 reduce quality?
Slightly. The audio in an MP4 is usually already compressed as AAC, and re-encoding it to MP3 is a second lossy step. At 192 kbps the difference is hard to hear. If you need no further loss at all, use MP4 to WAV or MP4 to M4A instead.
Can I convert several MP4 files at once?
Yes. Add as many as your device’s memory allows; the queue converts them in parallel where it is safe and gives you one ZIP at the end.
Does it work with MOV, MKV and other video files?
Yes. This page also accepts MOV, MKV, WebM, AVI, WMV, FLV and 3GP. Only the audio is ever written; no video file is produced.