About MP4 and M4A
MP4 is the standard video container on the web. Extracting its audio track is a quick way to turn a music video, lecture, or recording into a listenable file.
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 MP4 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.