Add Music to Video — Replace or Add Audio
Put a new soundtrack on a clip, or replace its audio entirely. The video stream is copied, not re-encoded, so it stays fast and keeps its original quality. No upload, works offline — your files are combined on your device and never leave it.
Runs entirely in your browser — your files are never uploaded.
- No upload Your video never leaves your device — it is processed locally.
- Works offline After the first load it runs with no internet connection.
- No size or daily limits No file caps, no paywalls — only your device memory.
- Private & safe Skip sketchy upload sites; nothing is sent to a server.
How it works
The job is handled by ffmpeg compiled to WebAssembly and run inside your
browser. Both files are loaded into memory, then ffmpeg maps the video stream from your clip and
the audio from your new track into a single MP4. Crucially it copies the video with
-c:v copy — no pixels are re-encoded, so your original quality is untouched and the
job is fast — and only encodes the audio to AAC at 192 kbps. In replace mode it maps just the new
audio; in mix mode it blends both with the amix filter. The -shortest
flag ends the output when the shorter of the two runs out, unless you choose to keep the full
video. Everything runs on your device, so nothing is uploaded, and the engine is cached after its
one-time download (about 9 MB) for offline use.
How to add music to a video
- Choose your video in the first drop zone, or drag it on.
- Choose the music or audio track in the second drop zone.
- Pick Replace or Mix, and choose how the lengths line up.
- Press Add music, then click Download when the MP4 is ready.
This is one of the fast tools because the video is copied, not re-encoded — only the audio is processed. Larger videos still take a little longer to read and write in the browser.
Why combine locally instead of on an upload site
Online “add audio to video” sites upload your full clip and your music to a server, combine them there and hand back a link — slower over a typical connection, and it exposes your footage and audio to a third party, often with size caps, watermarks or paid tiers. Doing it in your browser removes the upload step entirely, keeps both files private, and adds no watermark. It also sidesteps a genuine safety problem: in March 2025 the FBI’s Denver field office warned that some free online file tools were being used to spread malware — so keeping the work on your own machine is simply safer.
| Local (this site) | Typical cloud tool | |
|---|---|---|
| Upload required | No | Usually |
| File-size caps | None (device memory) | Common |
| Works offline | Yes, after first load | No |
| Files stay private | Yes — never sent | Sent to a server |
Common uses
Setting a song over a montage or highlight reel, replacing a clip’s noisy original sound with clean music, adding a voice-over recorded separately, putting background music under an existing narration with the mix option, or swapping the audio for a copyright-safe track before posting to social media. Trim your music first if you want it to start and end exactly with the video.
More video & audio tools
Frequently asked questions
Are my files uploaded to add the music?
No. Everything runs inside your browser with ffmpeg compiled to WebAssembly. Your video and audio are read into memory on your own device, combined there, and the finished MP4 is written locally for download. Nothing is sent to a server, so there is no upload wait and no copy of your files is stored online.
Why is this fast when compressing a video is slow?
Because this tool does not touch the video frames. It copies the existing video stream as-is and only encodes the new audio, which is light work. Compression, by contrast, re-encodes every single frame, which is heavy. So even a long video combines in seconds to a minute here, where compressing the same clip could take several minutes.
Does it replace the original audio or mix it in?
By default it replaces the original audio entirely with your new track — the most common use, like setting a song over a clip. There is also a mix option that blends your new audio with the video’s original sound, useful when you want background music under existing speech. Choose whichever you need before combining.
What happens if the music is longer or shorter than the video?
You control this. The default “match the shortest” option ends the result when either the video or the audio runs out, so there is no silent tail or frozen frame. The “keep the full video” option lets the video play to its end even if the music finishes first, leaving the remainder without the new track. Trim your audio first if you want it to line up exactly.
Why is the output always MP4?
MP4 with H.264 video and AAC audio is the most universally compatible format for phones, social platforms and players, so the tool muxes into MP4. Because the video stream is copied rather than re-encoded, your original video quality is preserved exactly; only the audio is encoded, to AAC at 192 kbps.
Does it work offline?
Yes. The site is a Progressive Web App, so it works without a connection after the first visit, and the ffmpeg engine is cached after its one-time download so combining keeps working offline.
All processing happens on Video Tools entirely within your browser. Your files are never uploaded to a server.