Add Audio to Video

Add audio or music to a video — without re-encoding the picture

This tool lets you add audio to a video in your browser: put a song under a holiday clip, replace a noisy soundtrack with a clean voice-over, or lay quiet background music under someone speaking. Choose whether the new audio replaces the original sound or is mixed with it, set the volume of each, decide when the music starts, and let it loop or fade out with the video.

Only the sound is rebuilt. The video stream is copied packet by packet into the new file, so the picture keeps its exact quality, resolution and frame rate, and the job finishes far faster than a full conversion. Your video and audio files are processed on your own device and never uploaded.

How to add music to a video

1. Drop your video into 1 · Video and the music or voice-over into 2 · New audio — MP3, WAV, M4A, AAC, OGG, Opus or FLAC.
2. Under Original sound, choose Replace it or Mix with the new audio, then set the volume sliders.
3. Set Start in the video at to delay the music, and Start the audio from to skip an intro in the audio file.
4. Choose whether short audio should loop until the video ends, and add a Fade in or Fade out. The timeline shows where the audio will play.
5. Press Add audio to video. The new video downloads automatically and appears in the result player.

Add Audio to Video features

Getting the levels right

When you mix, the two sounds are added together, so loud music can bury speech. For background music under talking, set the new audio to roughly 20–40% and keep the original at 100%. For a music video where the original sound is only ambience, do the opposite. Volumes above 100% amplify; if the sum gets too loud the peaks are clipped and the page warns you, so lower one of the sliders.

A fade-out of two or three seconds makes music end naturally with the video. If the music is looped, a short fade-in on the first pass and the fade-out at the end hide the start and stop; the joins between repeats are not cross-faded, so music that starts and ends on a beat loops best. Trim the music first with the Audio Cutter if you need a cleaner loop.

How it works

The page uses Mediabunny (MPL-2.0), an open-source media library loaded unmodified from a CDN, to read your video. A copy-only conversion moves the video packets into a new MP4, MOV, MKV or WebM container. Meanwhile the new audio is decoded with the Web Audio API, which resamples it to the video’s sample rate, and — in mix mode — the original soundtrack is decoded with WebCodecs. Both are combined second by second with your volume, offset, loop and fade settings and encoded with your browser’s AAC or Opus encoder.

Because audio and video are written in step, memory use stays modest even for long videos, although the finished file is assembled in memory before it is saved.

Related jobs

• Remove the original sound completely with Remove Audio from Video, which doesn’t need an audio file.
• Pull the soundtrack out of a video as an MP3 with Video to MP3, for example to reuse it in another clip.
• Shorten the video before adding music with Trim Video.