Add Audio to Video Online Free

Frequently asked questions

How do I add or replace audio in a video online?

Upload a video (MP4, WebM, or MOV) in the video zone and an audio file (MP3, WAV, or AAC) in the audio zone. Click "Replace audio". The tool uses FFmpeg.wasm to replace the video's audio track with your file. If the audio is shorter than the video, the output is trimmed to the audio length; if the audio is longer, it is trimmed to the video length. Download the result. Your files never leave your device.

Are my files sent to a server?

No. Processing runs entirely in your browser using FFmpeg.wasm. Your video and audio never leave your device.

What if the audio is shorter than the video?

The output duration matches the shorter of the two: the video is trimmed to the length of the audio. So you get a video that ends when your audio ends.

What if the audio is longer than the video?

The output duration matches the video. The audio is trimmed to the video length, so only the part that fits is used.

What video and audio formats are supported?

Video: MP4, WebM, MOV. Audio: MP3, WAV, AAC. The output is always MP4 (H.264 video, AAC audio) for compatibility.

Why does FFmpeg take a few seconds to load?

FFmpeg.wasm is a large WebAssembly module. It is downloaded once and cached. The first time you use the tool you will see a short loading state.

Does it work on mobile?

Yes, in browsers that support WebAssembly. Upload the video and audio from your device, then replace and download. No app install required.

Can I keep the original video audio and add a second track?

No. This tool replaces the existing audio with your file. For multiple audio tracks or mixing, you would need a desktop video editor.

Is there a file size limit?

There is no fixed limit. Very long or high-resolution files may take longer and use more memory. Practical limits depend on your device.

Does it work in all browsers?

FFmpeg.wasm requires a modern browser with WebAssembly. Chrome, Firefox, Safari, and Edge (recent versions) work.