Video Soundtrack puts another sound on top of a video or replaces the original one: silent time-lapse footage gets music, noisy travel footage is swapped for a BGM, a tutorial keeps its narration with light music underneath. It is the “Mux and soundtrack” tab of the audio/video workbench, and neither mode re-encodes the picture (-c:v copy), so quality is untouched and the job is fast.

“Replace audio” drops the video's original track and installs the file you dragged in; with “Loop the track when it is shorter than the video” enabled, a compatible track (AAC / M4A / MP3 / AC3 in an MP4) can also be stream-copied into the container in seconds. “Mix with original” uses amix to blend the original and the new track by two percentage sliders (100% original and 60% new track by default); the duration follows the video and the audio is re-encoded to AAC 192 kbps. The output container follows the source (MP4 / MOV / MKV / M4V, anything else falls back to MKV).

1. 01Add a video and select it— switch to the “Mux and soundtrack” tab.
2. 02Drop in the new track— MP3 / M4A / WAV / FLAC / OGG / OPUS all work.
3. 03Choose a mode— “Replace audio” or “Mix with original” (a video without a track can only be replaced); in mix mode drag the “Original volume” and “New track volume” sliders (0%–200% each) — the panel warns about clipping when the two add up past 250%.
4. 04Decide about looping— with “Loop the track when it is shorter than the video” the music repeats until the video ends; without it, silence fills the rest so the video keeps its full length. Then click “Replace track” or “Mix track” and download <original-name>_new-audio.mp4 or <original-name>_mixed.mp4.

Fixed start

— the new track always begins at 0 seconds; you cannot delay it or start from a chosen second of the music.

Hard-cut looping

— the loop returns to the start without a crossfade, so a rhythmic track will click at the seam, and the end does not fade out either (run “Fade in/out” on the result).

Mix mode only has volume ratios

— no auto-ducking (lowering music under speech), no EQ and no compression.

Containers and codecs

— MP4 / MOV / MKV / M4V output follows the source and anything else falls back to MKV; stream copy happens only for replace + loop + a compatible codec, otherwise the audio is re-encoded to AAC 192 kbps.

What it does not do

— it does not mix two audio-only files (use “Audio mixing”), does not adjust the original volume and export it alone (use “Volume adjust”), and does not remove a track (the mute mode of “Volume adjust”).

Video and audio are muxed locally in the browser by ffmpeg.wasm and nothing is uploaded. On first use the engine, about 31 MB, is downloaded from a CDN (the site asset domain or jsDelivr); that request does not contain your files.