Video to MP3

No. The engine is downloaded into your browser and runs there, reading the file straight from your disk.

The video engine is about 10 MB compressed. Your browser caches it, so it only happens once.

It copies the audio out of the video without decoding and re-encoding it. Nothing is lost beyond what the video had already lost, and you get an .m4a instead of an .mp3.

Keeping the original track reads only the audio out of an MP4, so a recording of several gigabytes goes through. The MP3 settings decode the whole file first, and a browser cannot read more than about 2 GB in one piece. You get a plain message when that happens.

No, closing the tab ends the job. You can switch away to another tab and come back; that does not slow it down.

Because the audio in that video could not go into an .m4a as it is. This happens with Opus audio, which is what videos saved from YouTube as WebM usually carry. Instead of failing, the tool converts it to an MP3 at 320 kbps, the highest setting it offers, and tells you it did. MP3 plays everywhere, including iPhone and Apple Music.

Yes. The picker takes any video your system lists as video. Choosing an MP3 bitrate sends the file through the video engine, which reads WebM, MOV and the rest and writes an MP3. Keeping the original track is fastest for an MP4 with AAC audio: the sound is copied straight out without the engine. Other files go through the engine for that option too.

No. The sound in a video has already been compressed once, and converting it to MP3 cannot bring back what was lost. A higher bitrate only keeps more of what is still there, at the cost of a bigger file. 192 kbps is the default. If you want no extra loss at all, choose to keep the original track, which copies the audio out without re-encoding it.

Yes. Add all of them before you press Start. They are converted one after another, not side by side, with a progress figure on each row, and you download them together as a zip. Each file keeps its own name with .mp3 on the end, or .m4a if you chose to keep the original track and the copy worked. If a file cannot be read as a video, its row says Failed and the next one still runs.