Replace Video Audio
Mute the original sound of a video and put a new audio track in its place. Runs in your browser, nothing is uploaded.
Everything runs in your browser. Your video and audio are never uploaded, and nothing is stored.
How to use it
Drop a video onto the first box and the new audio onto the second, or tap each box to pick a file from your device. Press Replace audio. The original sound is removed and your new track is added, then you get an MP4 to download. The video stream is copied without re-encoding, so there is no quality loss. If the audio runs longer than the video it is cut to match. If the video cannot be copied cleanly it is re-encoded as H.264. The first run downloads a 31 MB engine. That happens once and is then cached, so every run after it starts immediately.
Questions
Is my video uploaded anywhere?
No. The whole thing runs inside your browser using a WebAssembly build of ffmpeg. Your files never leave your device, which is why there is no size limit and no sign-up.
Why is the first run slow?
It has to fetch the 31 MB ffmpeg engine before it can do anything. The browser caches it, so the second run and every one after start straight away.
Does replacing the audio hurt the video quality?
No. The video stream is copied across untouched, so the picture is identical to the original. Only the audio is encoded, as AAC at 192 kbps.
What happens if the audio and video are different lengths?
The output stops at whichever is shorter, so a long song is trimmed to fit the video.
Which files does it accept?
Most common video formats such as MP4, WebM, MKV and MOV, and audio such as MP3, M4A, WAV and OGG. Very large files are held in memory, so a phone may struggle past a gigabyte or so.