Attach one or more audio tracks to a video: replace the original soundtrack, or mix music and narration over the existing sound. The video stream is copied untouched, so even long videos finish in seconds — and nothing is uploaded.
How to use Add Audio to Video
Drop in your video, then add one or more audio files.
Choose Replace audio (tracks play one after another as the new soundtrack) or Mix / overlay (tracks play on top of the original sound).
Click the action button, preview the result, and download the video.
Frequently asked questions
Will the video quality change?
No — the video stream is copied without re-encoding; only the audio is rebuilt.
What happens when the audio is shorter or longer than the video?
The output always keeps the video’s length: shorter audio is padded with silence, longer audio is cut at the end of the video.
Can I add several audio files at once?
Yes — in Replace mode they play in sequence as the new soundtrack; in Mix mode they are layered together (along with the original sound, if the video has any).
Is anything uploaded?
No — FFmpeg runs as WebAssembly in your browser, so your video and audio never leave your device.