MP4 to MP3
Keep the sound, drop the picture.
Your file
Drop a file here, or click to choose
It stays on this device. Nothing is sent anywhere.
Settings
What actually happens
An MP4 holds a video stream and an audio stream side by side. This discards the video, decodes the audio, and re-encodes it as MP3. That is why it finishes far faster than a video conversion: there are no pictures to process.
If you would rather not re-encode at all, choose M4A on the extract audio tool. Most MP4s already hold AAC, so M4A is the closest thing to lifting the audio out untouched.
The bitrate to ask for
The audio in an MP4 is usually AAC somewhere between 128 and 192 kbps. Since you are re-encoding rather than copying, the aim is to give the MP3 encoder enough room that it does not add anything on top of what is already there. Matching or slightly exceeding the source does that: 192 kbps is a safe default for music, and 128 is fine for anything spoken.
Going to 320 kbps is the mistake worth avoiding. It cannot recover what the video's encoder discarded when the file was made. All it does is store the existing result with great precision in a file twice the size. The only time 320 pays is when the source is lossless, which an MP4 never is.
Where the length comes from
The MP3 is exactly as long as the video, because the whole audio stream is converted. If you only want part of it, trim first rather than after: run the video through cut video or extract the whole thing and then use trim audio, which cuts to the millisecond and can put a fade on each end so the excerpt does not start and stop with a click.
What you cannot do here
There is no server on this site, so there is nothing that can fetch a URL. Every tool works on a file already on your device, which means an MP4 you have, not a link to a video hosted somewhere. That is a consequence of the whole design rather than a missing feature, and it is the same reason your file is never uploaded.
Batch conversion is also one file at a time for now. The command panel shows the exact FFmpeg line being run, though, and it is copyable: paste it into a terminal with a shell loop and you can apply the identical job to a folder of a thousand files.
Common questions
Is MP3 the best choice?
It is the most compatible. M4A keeps more quality at the same size, and is the better pick if everything you use supports it.
Can I convert several MP4s at once?
Not yet, one file per run.