Get the sound out of a video

Your video does not contain an MP3. Phones, cameras and almost every editor record AAC, which is a different and generally better codec of the same era. Every site that hands you an .mp3 from a phone video has decoded that AAC and compressed the result again — two rounds of lossy compression on sound that only ever needed one.

Writing that AAC into a file of its own instead means an .m4a. It is the same bytes the video was carrying, which is why it takes about a second and why nothing can be argued about its quality. If the sound genuinely is MP3 — some older cameras, some .avi and .flv files — you get a real .mp3 by the same route, and it is the best one obtainable.

When something you are feeding it will only accept .mp3: an old car stereo, a dedicated MP3 player, a podcast host or a form with a hard-coded extension check. Also when you need the file smaller and are willing to pay for that in quality — 128 kbps of a long lecture is a fraction of the size.

Otherwise the untouched copy is smaller and better, because AAC at a given bitrate beats MP3 at the same one. That is the whole reason the format exists.

The encoder is LAME — the same one behind almost every MP3 ever made — compiled to WebAssembly and loaded into this page only if you choose MP3. It runs on your processor. A long recording takes real time, and the page counts it out while it works rather than showing a spinner and hoping.

Converting needs a decoder for the original sound as well, and some browsers ship without the licensed AAC one. Where that is the case this page says so before you press the button, and the untouched route still works — it decodes nothing, so it needs nothing.

Before the download is offered, the page re-opens what it just made and reads its header back: the container, the codec, the sample rate, the channel count, the length, and that no video track came along by accident. Those are the numbers printed under the result. An earlier tool on this site announced that audio had been copied byte for byte while the converter was quietly re-encoding it, and every local test passed. A claim about a file is worth nothing unless something opens the file.

Recordings of meetings, lectures, interviews and voice notes are exactly the things people would rather not post to a stranger's server for the sake of a format change. This page has no server: the work happens in your browser, and the page's security policy blocks any connection to anywhere but this site. Here is how that is enforced.