How to get the sound out of a video
Drop in an MP4, MOV or WebM. Its soundtrack is decoded and drawn as a waveform, and the line under the file name says it is the sound of a video. Press Save MP3 for the whole soundtrack, or drag the handles first to keep a part of it: one song from a concert clip, one answer from an interview.
Which format to save
MP3 at 192 kbps plays everywhere and takes about 1.4 MB a minute. WAV keeps the decoded sound exactly, at about 10 MB a minute in stereo. Opus is smaller still. The sound in a video is almost always compressed already, so WAV cannot bring back what was lost; it only avoids losing more.
When a video won't open
Your browser does the decoding, so the sound has to be in a format it reads. AAC, the sound in most MP4 and MOV files, opens in every current browser; Opus and Vorbis, the sound in WebM, open in most. If yours can't read it, the page says so, and a video with no sound track says that too, rather than saving silence.
Questions
Is the video uploaded?
No. It is read in this page, on your device, which matters for footage you have not shared. The sound is held in memory while you work, about 20 MB a minute, so a phone may struggle with an hour of it.
Can it take the audio from a YouTube link?
No. It works on a video file on your device; it does not download anything from websites.
What about the File Converter?
The MP4 to MP3 converter does the same for a whole file in one step. Use this page when you want only part of the sound, a fade, or a change of volume.