Add Audio to Video
Put music or a voice-over on a video — mixed in or in place of its own sound.
Music, voice-over and the video’s own sound
Press play to hear the mix as it will be written — the video’s own sound and every added sound at its volume, start time and fades.
MP3, M4A, AAC, WAV, FLAC, Ogg/Opus and the sound of a video file. Up to three sounds, each up to 10 minutes.
What will happen
About the Add Audio to Video
Put a song, a voice-over or any sound file on a video. The new sound can be mixed with the video’s own sound — so the music sits under the speaking — or replace it completely. Each added sound has its own volume, start time, fade in, fade out, and can be repeated until the video ends or cut off when it does.
The picture is copied packet for packet: it is never decoded or re-encoded, so it keeps exactly the quality it had and the work takes seconds rather than minutes. Only the sound is written again, as AAC (MP4, MOV, MKV) or Opus (WebM) at 160 kbps.
Press play before you export: the page plays the video with the new sound in place, at the volumes and times you set, so you can hear the mix instead of guessing.
How to use it
- Drop a video on the box or click Choose a video. The picture appears with a player.
- Choose Music or voice-over to add — an MP3, M4A, WAV, FLAC, Ogg/Opus file, or even a video whose sound you want. Up to three sounds can be mixed.
- Set each sound’s Volume, where it starts, its fades, and whether it repeats until the video ends.
- Decide what happens to the video’s own sound: Keep and mix (with its own volume) or Replace it.
- Press play to hear the mix, then press Add the sound and Download the file.
Examples
2:30 MP4 with speech · music.mp3 at 25%, 1 s fade in, 3 s fade out · Keep and mix at 100%
2:30 MP4, picture untouched, one AAC sound track: the speech with the music under it
0:20 MP4 with no sound · song.m4a at 80%, starts at 0 s, repeats
0:20 MP4 with the song, cut off (with no fade of its own) when the video ends
1:00 screen recording · voice.wav at 100%, starts at 4 s · Replace the video’s sound
1:00 MP4 whose sound is silent for 4 seconds and then the voice-over
Mixing, not stitching
Mixing adds the sounds together sample by sample: every added sound is scaled by its volume and its fade, then added to what is already there, and the result is kept inside the range a sound file can hold (so it never wraps around into noise).
If the total is louder than that range, the loudest peaks are flattened — which is what clipping means. The page adds up the levels of your files and warns you when the mix is likely to clip, with a volume that leaves room.
Why the picture is untouched
A video file is a container with separate streams. Changing the sound means writing a new audio stream and copying the video stream across — the compressed picture data, exactly as it is. So:
- The picture keeps the quality, resolution, frame rate and codec it had.
- The file is written in seconds, even for a long video, because nothing is decoded.
- The result has one sound track, in the codec the container expects.
Which files you can add
Anything this browser can decode: MP3, M4A/AAC, WAV, FLAC, Ogg/Opus, AIFF, and the sound of a video (MP4, MOV, MKV, WebM). Each added file is read into memory, so files are limited to 10 minutes each and 20 minutes in total — cut longer music first with the MP3 Cutter.
The video itself is read from the file on your device as it is written, so its length is not limited in the same way.
Limitations
- One sound track in the result: added sounds and the video’s own sound are mixed together, not kept as separate tracks to choose from.
- Mixing with the video’s own sound needs a browser that can decode it and encode AAC or Opus (current Chrome, Edge, Safari or desktop Firefox). Replacing the sound works wherever the picture can be copied.
- Added files are held in memory: up to 10 minutes each, 20 minutes together.
- Videos with several sound tracks keep only the first one; the others are left out.
- There is no automatic ducking yet (the music is not lowered by itself under speech); set the music volume to about a quarter for speech under music.
- The mix you hear on the page is played by the browser and can drift by a few hundredths of a second; the written file is sample-accurate.
Privacy
Everything happens in your browser. What you enter or open here is not uploaded or stored by MySmartCoPilot.
Frequently asked questions
Will the video lose quality?
No. The picture is copied as compressed data without being decoded, so it is bit-for-bit the same as in your file. Only the sound is encoded again.
Can I keep the original sound and add music under it?
Yes — that is Keep and mix. Set the video’s own sound to 100% and the music to about 20–30%, so speech stays clear. The page warns you if the two together would clip.
What if the music is shorter than the video?
Turn on Repeat until the video ends and it loops; otherwise it simply stops where it ends. A fade out always lands at the end of what is heard.
What if the music is longer than the video?
It is cut off when the video ends. Choose a start time to pick which part of the song is heard, or cut the song first with the MP3 Cutter.
Is my video or music uploaded?
No. Both are read on your device, mixed in your browser, and the result is saved straight to your device.
Which sound codec does the result use?
AAC at 160 kbps in MP4, MOV and MKV files, Opus at 160 kbps in WebM — the codecs those containers are expected to carry, so the file plays everywhere.