Are my files uploaded when I add audio to a video?
No. Local video and audio are decoded, mixed, and re-encoded entirely in your browser. URL mode only fetches the file from its original server.
Which input formats are supported?
Video: MP4, WebM, MOV, MKV. Audio: MP3, WAV, M4A, AAC, OGG, Opus, FLAC. The browser must be able to decode the codec used inside the file.
What is the output format?
The result is an MP4 file with H.264 video and AAC audio when your browser supports them, falling back to the first encodable codec available.
How does "Add as background" differ from "Replace original" and "Voice-over"?
Background mixes the added audio with the original at full volume. Replace silences the original entirely. Voice-over keeps the original at about 30% volume so the added track sits clearly on top.
What does "Loop audio if shorter" do?
When the added audio is shorter than the selected range, the same audio loops seamlessly until the range is filled. It is useful for short background music or ambient loops.
What does the normalize option do?
Normalization scans the final mix and scales the loudest peak to a safe level (about -0.5 dBFS) so the output is loud enough without clipping.
Can I trim the added audio?
Yes. Set Start time and End time under Audio Settings. Only the selected portion is mixed into the video and the rest is discarded.
Why does the result match the original video duration?
The video timeline is the source of truth. If the added audio is longer than the video, anything past the video duration is cut. If shorter, it stops naturally or loops when looping is enabled.
Does the original video keep its quality?
The video is re-encoded so the new audio can be muxed into a clean MP4. Quality is preserved as much as the browser encoder allows; lower-resolution sources stay close to their original sharpness.
Why does a remote URL fail?
Remote sources must be a direct media URL that allows browser CORS plus Range requests. Streaming pages, signed-only CDNs, and download portals usually do not.
Will my original files be modified?
No. Both source files remain untouched. The tool produces a brand new MP4 you download separately.
Why can a very large video fail?
Browser memory and codec support are the practical limits. Smaller files, common MP4 sources, and updated browsers (Chrome, Edge, Safari, Firefox) work most reliably.