Add Audio to Video
What this tool does
Pick a video and an audio file, and it combines them into a single MP4 with your chosen soundtrack. It is the quick way to put background music under a clip, drop a voiceover onto a screen recording, or swap out a video’s existing audio. The new audio replaces whatever sound the video had, cut to match the video’s length.
No quality loss
The video’s picture is copied across exactly as it was — the tool never re-encodes the video, so there is no generation loss and the combine is fast even for long clips. Only the audio is encoded, to AAC, and both tracks are packaged into a fresh MP4 that plays everywhere.
Private and on-device
All of this happens locally using your browser’s built-in video codecs (WebCodecs) — no server, no upload, and no giant engine to download. It works best with an MP4 or MOV video (H.264 or H.265) plus an MP3, WAV, M4A, or OGG audio file, in an up-to-date Chrome, Edge, or Safari.
Frequently asked questions
Are my files uploaded?
No. The video and audio are combined entirely in your browser using its built-in video codecs (WebCodecs), so neither file ever leaves your device.
Does it re-encode and degrade my video?
No — the video track is stream-copied as-is, so there is no quality loss and it is fast. Only the audio is (re-)encoded, to AAC, and the two are muxed into a new MP4.
What happens to the video’s original audio?
It is replaced. The result contains your new audio track only. The new audio is trimmed to the video’s length, so the file ends with the video.
What formats work?
Video: MP4 or MOV using H.264 or H.265. Audio: MP3, WAV, M4A/AAC, or OGG. The output is always an MP4.
It says my browser can’t encode audio — why?
Combining requires the WebCodecs AAC encoder, which the latest Chrome, Edge, and Safari support but some browsers (or older versions) do not. Try an up-to-date Chrome, Edge, or Safari.