About WebM and M4A
WebM is the open web video container used by browser screen recorders and much of YouTube. Its audio track is typically Opus or Vorbis.
M4A is AAC audio in an MP4 container — the default format of the Apple ecosystem and most streaming services. AAC compresses more efficiently than MP3, so the same bitrate sounds a little better.
Extracting audio from WebM re-encodes only the audio track — the video is discarded, so even long files finish quickly.
How the conversion works
This tool runs FFmpeg — the same engine behind most media software — compiled to WebAssembly, entirely in your browser. Your file is read locally, converted locally, and saved straight from memory; nothing is ever uploaded, which also means no queues and no server-side file size limits.
The converter itself is a one-time ~31 MB download on your first conversion, cached by the browser afterwards. Once it's loaded, everything works offline.