Convert MP4 to MP3
Keep the sound from a video and throw the picture away.
How it works
An MP4 is a container, not a codec. It holds separate numbered tracks, each with its own codec, and a table of offsets saying where every chunk of each track sits in the file. Reading one means parsing that table, walking the audio track's chunks, and ignoring the video track's entirely. The video data is never decoded here, so the size of the picture has almost no bearing on how long this takes.
The audio inside is nearly always AAC. That is what phones, cameras, screen recorders and every video site produce, because AAC was designed as MP3's successor and beats it at the same bitrate. MP3 is the older format and the one that plays on hardware AAC never reached.
This is a re-encode, and that costs something
Because the source is AAC and the destination is MP3, the audio has to be decoded to raw samples and encoded again. Two lossy formats in sequence means two rounds of discarding. Each codec throws away a different set of detail according to its own psychoacoustic model, so the second pass cannot know what the first already removed and spends bits describing artefacts instead of music.
The practical defence is headroom. Encoding to a higher bitrate than the source used gives the MP3 encoder room to reproduce what it was handed, including the flaws. A 128 kbps AAC track re-encoded at 192 kbps MP3 sounds close to the original; the same track at 96 kbps sounds worse than either.
What comes out is audio only
The video track is discarded rather than hidden, so the result is a plain audio file with no picture in it and no way to get one back. Chapter markers, subtitles and any second audio track in another language go the same way, because MP3 has nowhere to put them.
Track titles and artist tags are carried across where the source file has them. Most screen recordings and phone videos have none, so an MP3 made from one arrives untitled and takes the video's filename.
Long recordings are fine. A two-hour lecture is a few hundred megabytes of MP4 and comes out around 110 MB at 128 kbps, and because only the audio track is touched the conversion is far quicker than anything that has to look at the frames.
Questions
Is my video uploaded anywhere?
No. Open developer tools, switch to the Network tab, and run a conversion. You will see same-origin requests for the page's own code and for the LAME encoder module, then nothing further. The video is read by the page, parsed in a Web Worker on your machine, and the MP3 is handed back as a download. Nothing carrying your file leaves the tab.
Which video files work?
MP4, M4V, MOV, WEBM, MKV, AVI and MPEG transport streams, provided the audio inside is a codec the browser can decode. AAC, MP3, Opus, Vorbis, FLAC and raw PCM all qualify. AC-3 and E-AC-3, the Dolby tracks common in ripped films, do not: browsers omit those decoders for licensing reasons and the page says so rather than producing silence.
Which bitrate should I pick?
192 kbps for music from an ordinary video, because it leaves room for the second encode. 128 kbps if the source is already a low-bitrate stream and the result is going on a phone. 96 kbps or 64 kbps mono for speech, which is what a lecture, an interview or a podcast recording amounts to, and where the saving is large and inaudible.
Can I take the audio from a YouTube video?
Only if you already have the file. This tool works on a video on your disk and has no way to fetch one from anywhere, by design: the page makes no network requests once it has loaded. Downloading from a streaming service is a separate question, and one governed by that service's terms rather than by what a converter can do.