Add Audio to Video is a free, in-browser video tool. Add a music or voiceover track to any video — mix it on top of the existing audio or replace the audio entirely. Files are processed locally in your browser with FFmpeg WebAssembly. The page exposes a small surface — input, controls, output — so a first-time visitor can complete the job without reading documentation.
Add Audio to Video fits naturally into the workflow of teams compressing demo recordings and social-media managers cutting reels, both of whom typically need a fast result inside the browser. There is no learning curve to budget for: anyone who has used a typical web upload form can complete a run on the first try.
Add Audio to Video is shaped for the gap between "I'll do it by hand" and "I'll script it." When the job is small enough that automating it would take longer than doing it, but annoying enough to want a focused tool — that is the situation this page is built for.
The engine behind the page is FFmpeg compiled to WebAssembly. It reads your file in-memory and writes the result back into the browser. Supported inputs include MP4, WebM, MOV, MKV, AVI, FLV, and OGV. For 500 MB and below the work usually completes in seconds; larger files mostly depend on how much spare RAM your device has.
Add Audio to Video is structured so the question "where is my file processed?" has a single answer: in your browser tab. The engine, the controls, and the result panel are all on one page. Navigating away or closing the tab clears the page's memory the way it does for every other tab.
Add Audio to Video is intentionally narrow in what it does, which makes it easy to slot into a longer workflow. Take its output, hand it to whichever next tool fits the job, and Add Audio to Video stays out of your way until the next time you need it.
The architecture imposes only the limits the browser itself imposes. The published 500 MB ceiling is conservative; most modern devices comfortably handle inputs up to that size, and the cap exists so the tool degrades gracefully on phones and budget laptops rather than running out of memory.
The transformation in Add Audio to Video is deterministic — the same input plus the same options produces the same result every run. That predictability matters when the result has to match an upstream specification or be reproducible later.
Output handling is intentionally boring: Add Audio to Video produces `{name}-edited.{ext}` and triggers your browser's standard "save" behaviour. If you have a default download folder configured, that is where it will land. There is no Favtoo-side history of jobs you have run.
Add Audio to Video is structured around the idea that a useful tool should be its own page. Open the page, do the work, close the tab — the page is the entire product. There is no onboarding flow because there is nothing to onboard into.
Add Audio to Video is built around the moment of need: a focused page you open when you have a specific task, complete the task, and close. The catalog contains many adjacent tools so the same model serves the surrounding parts of a typical video editing and conversion workflow.
Tips from users who reach for Add Audio to Video regularly: process one input first to confirm the settings produce what you expect before committing to a batch; treat the page as the working surface and avoid leaving large jobs running in a backgrounded tab where the browser may throttle JavaScript; and if a particular file fails, check whether the source is intact by opening it in its native viewer — most "tool errors" are actually input errors.
If Add Audio to Video appears to hang, the engine is almost certainly still working — large inputs simply take longer to process inside a browser than they would on a server with multi-core scheduling. For inputs near the 500 MB cap, give it up to a minute on a typical laptop before assuming something is stuck.
That is the whole tool. Use Add Audio to Video for as long as it stays useful to you, and if it does, the catalog has many more tools built the same way. Each applies the same single-purpose discipline, so the way you used this page transfers to the next one you try.
A WebAssembly build of FFmpeg loads in this page and combines your video and audio files entirely on your device. The video stream is copied without re-encoding (fast), and the audio is mixed or replaced into a new MP4.
Server tools use native multi-threaded FFmpeg with hardware acceleration. WebAssembly is single-threaded inside the browser, typically 2–5× slower for audio mixing. The trade-off is total privacy: your files never leave your device.
MP3, WAV, AAC, M4A, OGG, and FLAC. The audio is automatically re-encoded to AAC at 192 kbps for the MP4 output, which plays everywhere.
Replace removes the original video audio entirely and uses your new audio in its place. Mix layers your new audio on top of the existing audio at equal volume — useful for adding background music to a clip with narration.
The output is automatically trimmed to the shorter of the two streams (`-shortest`). If your audio is longer than the video, only the first portion is kept. If shorter, the trailing video has silence.
Up to 500MB of video and 100MB of audio. Because the video stream is copied without re-encoding, even very long videos process quickly.
No. The video stream is copied byte-for-byte into the new MP4 container. Only the audio is re-encoded to AAC. Visual quality is identical to your original.
No. The combination runs entirely in your browser tab. Nothing is transmitted, stored, or logged. Closing the tab erases the files from memory.
Add Audio to Video works in any modern browser released in the last few years — Chrome, Edge, Firefox, Safari, Brave, Arc and the major Chromium derivatives are all supported. The underlying engine relies on widely-supported web APIs, so there is nothing exotic to install. If you are on a very old browser version and the tool fails to load, updating to the latest release of your preferred browser is the only fix needed.
Inputs are capped at 500 MB per file, which keeps memory usage stable across phones, tablets and older laptops. You can run Add Audio to Video as often as you need; every run produces a full-quality result.
Add Audio to Video is built to preserve quality wherever the underlying video format allows it. Operations that are mathematically lossless (e.g. structural transformations, lossless re-encoding) round-trip with no perceptible change. Operations that involve a lossy codec inevitably introduce small artefacts at the byte level, but the defaults aim at the sweet spot where output looks or sounds the same to a normal viewer or listener while still being meaningfully smaller or faster than the input.
Add Audio to Video is a browser-only tool by design and does not expose a hosted API. The reason is the same as the privacy story: there is no Favtoo backend doing the work, so there is no service to call. If you need to script the same transformation, the underlying engine (FFmpeg compiled to WebAssembly) is open-source and can be used directly from your own code.
No installation is needed. Add Audio to Video runs as a normal web page, with no browser extension, no native helper, and no separate desktop client to download. That is partly a privacy choice — extensions can request broad permissions, while a regular page is sandboxed by default — and partly a convenience one: you can use Add Audio to Video on any computer you have temporary access to without leaving anything installed on it.
Add Audio to Video is built on FFmpeg compiled to WebAssembly, which is the same class of engine used by professional video editing and conversion pipelines. For deterministic operations, the output is byte-identical to what an equivalent CLI run would produce; for operations involving a codec or a model, the result is well within the range of what comparable tools generate. If you have a specific reference output you need to match, run a small test job first to confirm the configuration produces what you expect.
Add Audio to Video is free to use. The processing runs in your browser, which keeps the per-user cost low enough that the tool can be offered openly. The download is the same file the engine produced — you can use it for as many runs as you need.
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