Add Audio to Video
Put music, a voiceover, or the sound from another clip onto a video — replacing what is there or mixing with it. The picture is copied rather than re-encoded, so the file you get back is the one you brought, with a new soundtrack on it.
Why your picture never loses quality here
Most browser tools that "add audio" re-encode the whole video to do it, because that is what a general-purpose conversion pipeline does. It is not necessary. A video file holds a picture track and a sound track side by side, and swapping one has nothing to do with the other: the frames can be copied across exactly as they are, one compressed packet at a time, while a new soundtrack is written beside them. That is the only thing this tool does to your picture — on every setting, including mixing, looping and offsets.
The sound is a separate question, and the tool answers it out loud before you press export. An MP3 or an M4A dropped onto an ordinary MP4 is copied too, so nothing in the file is decoded at all and the export takes about as long as reading the two files. A WAV, a FLAC, a mix, a loop or a level change all mean the soundtrack has to be built rather than copied — still fast, since audio is a fraction of a video's data, and still with the picture untouched. Whichever of those you are getting is on screen, with the reason, before anything runs.
If you want the opposite of this page, removing a soundtrack is a stream copy for the same reason. To lift a soundtrack out of one video before putting it on another, start with extracting the audio — though you can also drop a video straight into the audio slot here and this tool will take its sound. And if what you actually want is to join clips end to end, merging videos is the tool for that; it is a different operation, which is why it has its own page.
Frequently asked questions
How do I add audio to a video in the browser?
Drop the video in the first slot and the audio file in the second, press play to hear the two together, then export. Nothing is uploaded: both files are read from your disk and joined inside this browser tab.
Does adding a soundtrack re-encode the video?
No — and not just usually. Putting sound on a picture never touches a video frame, so the picture is copied packet for packet on every path this tool can take. That is why an export finishes in about the time it takes to read the files, and why the result is bit-for-bit identical to the video you started with.
Can I keep the video's original sound as well?
Yes. Choose “Keep both, mixed” and the two soundtracks play together, each with its own level — the usual choice for putting music under someone talking. Replacing is the default because the most common reason to add audio is that the video has none, or has the wrong one.
What happens if the music is longer than the video?
You choose, and the timeline shows it before you commit: cut the track where the picture ends, loop it to fill the picture, or keep it whole and let it run past the last frame. The tool says in words what that means for your two files — for example that the track is 2:31 longer and will be cut.
Can I nudge the sound to line up with the picture?
Yes, there is a start offset in hundredths of a second. A positive offset holds the track back after the first frame; a negative one starts it partway in, which is what you want when the audio was recorded on a separate device and started rolling first.
Which audio formats can I use?
MP3, M4A/AAC, WAV, FLAC and OGG, and you can also drop in a video file to borrow its soundtrack. An MP3 or M4A on an ordinary MP4 is a pure stream copy — nothing is decoded at all. A WAV or FLAC is re-encoded to AAC, because an MP4 carrying either of those plays almost nowhere; the picture is still copied.
Is this the same as merging videos?
No. This puts one soundtrack onto one picture — muxing. Joining clips end to end is a different operation with a different tool: use the video merger for that.
How big a file can I use?
Up to 1 GB for the two files together. The result is built whole in this tab's memory before it downloads, which is what sets that ceiling.
Does anything get uploaded?
No. There is no server, no queue, no account and no watermark — both files are read from your device and the result is written in this tab.