Select a video file, wait for its audio track to be decoded, then press Extract Audio. The extracted sound appears in a player and downloads as a WAV file.
Anything whose audio track your browser can decode, which in practice covers most MP4/AAC, WebM/Opus and many MOV files. If a file fails, the container or codec is not supported by your browser and a different browser may succeed.
No. The file is read locally and only its audio stream is decoded with the Web Audio API. Nothing is transmitted, which matters when the source is private footage, an interview or unreleased material.
Browsers do not ship an MP3 encoder, so writing WAV directly from the decoded samples is the only way to guarantee a correct file without third-party libraries. You can compress the WAV afterwards in any editor if you need a smaller file.