Put a song or voice-over on a video to the millisecond, with the picture untouched
Processed in your browser. Nothing is uploaded. Updated on 24 Oct 2026.Drop a video and an audio file, slide the audio into place to the millisecond, and save. Every video frame is copied untouched, so a 4K clip keeps its quality and a 2 GB file finishes in under a minute. The sound is copied too when it is AAC heard as it is; otherwise it is encoded at 256 kbps.
Press Choose video and pick an MP4, MOV or M4V file; you can drop the audio file with it at the same time.
Press Choose audio and pick an MP3, M4A, WAV, OGG or FLAC file.
Set where the audio begins with Start the audio at, and set the Original sound and New audio volumes.
Press Save the video with this sound and choose where to save the MP4, or let it download. The picture is copied untouched and nothing is uploaded.
This page puts a new sound track on a video: a song, a voice-over, a cleaner microphone recording, or nothing at all. It was built for the job of finishing AI-generated clips for YouTube, where the picture is final and only the sound changes, and for that job the one thing that must not happen is the picture getting worse.
So the picture is never re-encoded. Every video frame is read from your file and written into the new one byte for byte, which is why a 4K or 8K video keeps every pixel and why a long file finishes quickly: nothing is decoded, nothing is compressed a second time. The quality check behind this page compares the frames of the result with the frames of the original and requires them to be identical.
Lining it up. The waveform of the new audio is drawn under the video, and the video's own sound above it when it has one, so a clap, a beat or the first word can be matched by eye. Slide the offset or type it in milliseconds: a positive offset starts the audio later, a negative one skips its first seconds. Press play to hear the two together before saving.
Longer or shorter than the video. Audio that runs past the end of the video is cut at its last frame. Audio that ends early leaves silence, or starts again from the top if you choose Repeat. A fade in and a fade out are there for music.
Keeping the original sound. Set its volume above zero and it stays, mixed under the new track at the level you choose; at zero it is replaced.
How the sound is written. When the new audio is AAC in an M4A or MP4 file and is heard exactly as it is from the first frame, its frames are copied too, and the whole file is lossless. Anything else is mixed and encoded to AAC at 256 kbps, which is transparent to the ear; the page says which, and why, before you press the button.
Nothing is uploaded. In Chrome and Edge the result is written straight to the place you choose on your disk, so even a very large video needs no extra memory; other browsers download it.
Drop the video and the music file here and save. The video frames are copied into the new file exactly as they were, not re-encoded, so a 1080p or 4K video keeps every pixel. Only the sound track is new.
Drop the video and the new audio, leave "Original sound" at 0%, and save. The old sound track is dropped and the new one takes its place, lined up where you put it and cut at the video's last frame if it is longer.
Use the offset: a positive number starts the audio later, a negative number starts it earlier by skipping its first part. The waveforms of both tracks are drawn one above the other so you can match a clap or a word, then press play to check the lips before saving. Steps of 10 milliseconds are fine enough for lip sync.
Yes. Set Original sound above 0% and the video's own sound stays, with the new audio mixed in at its own volume. For background music under speech, 100% original and 20 to 30% music is a common starting point; a fade-out at the end avoids stopping mid-note.
MP3, M4A, AAC, WAV, OGG, Opus and FLAC, and the sound of another MP4 or MOV. The video must be MP4, MOV or M4V. The result is always an MP4 that plays everywhere and uploads to YouTube as it is.
No. Both files are read inside your browser and the new video is written on your own device. That is also why there is no size limit beyond your disk in Chrome and Edge, where the file is saved as it is written.
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Also works as: Sync Audio with Video.