FLV → MP3
FLV to MP3 converter
The AAC or MP3 audio inside the FLV is decoded and encoded to MP3 with LAME; the video is never touched. Both source codecs are lossy, so keep the bitrate at or below the original.
…or drag files and folders here. Add as many as you like — everything is processed on this device.
.asf .avi .f4v .flv .m4v .mk3d .mka .mkv .mov .mp4 .qt .wmv
In one paragraph
FLV files are a leftover of the Flash era: saved web videos, old stream recordings, and some RTMP capture tools still write them today. Flash is gone from every browser and many players no longer open FLV, but the audio inside is ordinary AAC or MP3, which makes it easy to rescue.
Sonic Batch opens the FLV with a WebAssembly build of FFmpeg inside your browser, reads the audio stream and writes a new MP3. Nothing is uploaded, which matters for stream recordings: they are often hours long and hundreds of megabytes, and here the only limit is what your device’s memory can hold.
Does this change the audio quality?
FLV audio is already lossy (AAC or MP3, usually 64–128 kbps). Encoding to MP3 adds a second lossy generation, so a higher target bitrate cannot improve it; match the source.
- Very old FLV files may use Nellymoser or Speex voice codecs. They decode, but sound as limited as they did in Flash.
- Stream recordings over 400 MB are rejected before decoding. Trim or split them first.
- Only the first audio track is extracted.
A worked example
Keeping the audio from an old saved web video
- Drop the FLV file into the queue.
- Leave the bitrate at 128 kbps, which matches most Flash-era video.
- Add a title and artist in the tag editor.
- Convert and download the MP3.
128 kbps is right for nearly every FLV
Most Flash video was encoded with 64 to 128 kbps audio to save bandwidth, so a 128 kbps MP3 holds everything that is there. Use 96 kbps mono for talk and stream chat recordings. Raising the bitrate above the source only enlarges the file, because the original encoder already discarded the detail.
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 |
|---|---|---|---|---|---|
| FLV | Flash-era web video, still produced by some stream recorders. Audio is AAC or MP3. | Engine | Engine | Engine | Engine |
| MP4 | The standard video container. Its audio track is almost always AAC. | Native | Partial | 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 |
| 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 |
| MOV | QuickTime video from iPhone and DSLR cameras. Audio is normally AAC or PCM. | Partial | Engine | Native | Native |
| 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
The audio is out of sync or cuts off early.
Stream recordings that ended abruptly often have a broken final section. The audio up to that point converts normally; try trimming off the last few seconds.
It says no audio stream found.
Some FLV files carry video only. Play the file in VLC to check; if it is silent, there is nothing to extract.
What about F4V files?
F4V is the later MP4-based Flash format. It is accepted here and converts like an MP4.
Frequently asked questions
How do I convert FLV to MP3?
Add your FLV files above, choose a bitrate, and press Convert all. Download individually or as a ZIP.
Do I need Flash to convert FLV?
No. The engine reads the FLV container directly. Flash is not involved.
Are my files uploaded?
No. All processing runs in your browser tab and nothing leaves your device.
Can I get the audio without re-encoding?
Not on this page. It always writes a fresh MP3, which costs one lossy generation; at a matched bitrate that is very hard to hear.