Recorded a perfect screen capture with imperfect audio? Drop the video on the timeline, delete the original channel, add your new narration or music, nudge it into place, and export. The video itself is copied byte-for-byte — zero quality loss, seconds not minutes — and nothing is uploaded.
Checking local library…
Online "video converters" re-encode your footage — slow, lossy, and pointless when all
you want is different audio. This timeline does what
ffmpeg -c:v copy does: the picture stream moves into the new file
byte-for-byte. We verified the exported video stream is bit-identical to the source.
Take your voiceover in the free voice recorder — it lands in your local library, and the "From library" button drops it straight onto this timeline. Trim or polish it first in the trimmer or studio.
The companion audio extractor pulls the existing track out of a video as an exact-copy M4A or an editable WAV — same engine, same privacy: nothing leaves your machine.
No — that is the whole point. The video stream is copied byte-for-byte into the new file, exactly like ffmpeg's "-c:v copy". Only your new audio is encoded (to AAC, the standard for MP4). Export takes seconds, not minutes, and the picture quality cannot change.
The video: MP4 or MOV with H.264 video — what macOS/Windows screen recorders and most phones produce. The replacement audio: anything your browser can decode — WAV, MP3, M4A, FLAC, OGG — or any take from your local library, including ones you record in the free voice recorder.
No. The demuxing and rebuilding is a small WebAssembly module compiled from Rust and served with this page; the audio encode uses your browser's built-in encoder. The file is processed in this tab's memory and handed back. No server, no queue, no account.
Yes — the offset control slides the replacement channel along the timeline (for example, start the narration 2 seconds in). The gap is filled with silence, and anything past the end of the video is trimmed.
The export always carries only your replacement channel — delete the original channel on the timeline to make that explicit. Your original file on disk is never modified; you always download a new file.
Chrome, Edge, and Safari 17+ (they ship the WebCodecs AAC encoder). In browsers without it — notably some Firefox versions — the timeline still works but the export button explains what is missing instead of failing silently.