Add Audio to Video
Replace or mix a soundtrack onto a video in your browser and download the result. Free, private, no signup or upload.
About the Add Audio to Video
This free tool adds a soundtrack to a video — either replacing the existing audio entirely or mixing your new track in with it. Use it to set a video to music, add a voice-over, or drop in a sound effect. Provide the video and the audio file and the tool combines them into a single MP4.
Everything runs in your browser using FFmpeg WebAssembly, so neither the video nor the audio is uploaded. The engine (about 30 MB) downloads from a CDN on first use and is cached, so the first run is the slow one. In replace mode the video stream is copied untouched for speed; in mix mode both audio tracks are blended together.
The result is trimmed to the shorter of the video and audio, so if your music is longer than the clip it will stop when the video ends. For the reverse — pulling audio out of a video — use our Extract Audio from Video tool. Files up to 100 MB each are supported.
How to Use the Add Audio to Video
- 1Choose a video file, up to 100 MB.
- 2Choose an audio file (MP3, WAV, M4A and more).
- 3Pick Replace (swap the audio) or Mix (blend with existing sound).
- 4Click Add Audio, preview and download the MP4.
Frequently Asked Questions
Is my file uploaded to a server?
No. All processing runs in your browser using FFmpeg compiled to WebAssembly. Your file never leaves your device — only the converter engine is downloaded, once.
Why is the first run slow?
The first time you use any converter here, the browser downloads the FFmpeg core (about 30 MB) from a CDN and compiles it. It is then cached for the session, so later runs are much faster.
What is the difference between replace and mix?
Replace swaps out the video's original sound for your new track. Mix keeps the original audio and blends the new track on top — useful for adding background music under speech.
What if the audio is longer than the video?
The result is trimmed to the shorter of the two, so extra music beyond the end of the video is cut off. Trim your audio first if you want a specific ending.
Can I remove a video's audio instead?
Yes — use our Mute Video tool to strip the sound, or Extract Audio from Video to save the soundtrack as a separate audio file.