Video Audio Sync

What is Video Audio Sync?

A free online tool for replacing or mixing a video's audio with separately recorded tracks, entirely in your browser. Video creators, vloggers, interviewers, and musicians often capture clean sound on a dedicated recorder while filming, then have to line it up with the footage and decide which audio to use where. This tool extracts the video's built-in (scratch) audio and cross-correlates it against each external recording to estimate the offset in milliseconds, with a confidence score per track. You then build a segment timeline — choosing the original audio, an added track, or silence for each region, adjusting where each track plays, and adding fade in and out — and export an MP4 with the video copied untouched and the new audio mix muxed in. Because it runs on ffmpeg compiled to WebAssembly, detection, mixing, and muxing all happen on your device.

How to use Video Audio Sync

1. Drop your video onto the first dropzone, then drop one or more separately recorded audio tracks onto the second. The first run downloads the in-browser engine once.
2. Wait while the tool reads each track and auto-detects its time offset against the video's sound, shown with a confidence score.
3. Adjust an offset if needed (positive delays the track, negative trims its start), then click Apply to realign that track’s segments — or use Reset offset to return to the detected value.
4. Under Segments, add regions and pick a source for each: the original camera audio, one of your added tracks, or silence. Drag each region within the highlighted band that shows where the chosen track actually plays.
5. Optionally set a fade in and fade out per region, and turn on Preview the mixed result to compare the mix against the original as it plays.
6. The synced video rebuilds automatically — play it to check, then click Download to save the MP4.

Examples

Frequently asked questions

How does the automatic sync work?

The tool extracts the video's own audio and cross-correlates its waveform against each external recording to find the time offset that best lines them up, reporting a confidence score per track so you know whether to trust it or adjust it.

Can I use more than one track and choose which one plays when?

Yes. Add as many tracks as you like and build a segment timeline: for each region choose the original camera audio, any added track, or silence. Regions you don't cover fall back to the original audio.

Can I fade the audio in and out?

Yes. Each segment has its own fade in and fade out, in seconds, applied precisely in the exported MP4 and approximated in the live preview.

What if the detected offset is wrong?

Auto-detection can struggle when a track shares little common sound with the video, or on long clips where device clocks drift. Type the offset by hand and click Apply to realign that track’s segments, use the preview to verify, or Reset offset to go back to the detected value.

Does it re-encode my video, and are my files uploaded?

The video stream is copied as-is with no re-encoding — only the audio is mixed and encoded to AAC in an MP4. Detection, mixing, and muxing run 100% client-side with ffmpeg in WebAssembly, so your files never leave your device.