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Add audio to video

Drop a video, choose an audio track, and download the result, everything runs in your browser.

How Add audio to video works

Attach a soundtrack to a silent video or replace an existing one entirely in your browser, with ffmpeg WebAssembly muxing the streams on your device. Drop a video and an audio file (MP3, WAV, M4A, AAC, OGG, and more are all accepted), choose whether to replace the original sound or mix both tracks together, set the added track's volume relative to the original, and decide what to do when the two streams differ in length: stop at the shorter one or loop the audio to cover the full video duration.

The video stream is copied with -c:v copy, so no pixel is re-encoded and quality is lossless. Only the audio track is encoded to AAC, then trimmed to length with -shortest. The result is an MP4 that keeps your original video resolution, frame rate, and visual quality intact.

Written by Bastien Sulyan

How to use Add audio to video, step by step

  1. Drop or select the video file you want to add audio to.
  2. Drop or select the audio file (MP3, WAV, M4A, OGG, or AAC).
  3. Choose Replace to remove the original audio, or Mix to layer the new track over the existing one.
  4. Adjust the volume of the added track if needed.
  5. Select whether to stop at the shorter stream or loop the audio, then click Add Audio and download the result.

Common use cases

  • Adding a royalty-free background music track to a silent screen recording before publishing a tutorial.
  • Replacing the wind noise from an outdoor drone clip with a clean narration recorded separately.
  • Mixing a sound effect layer over a game-capture video to increase impact on social media.
  • Attaching a separately recorded voiceover file to a slide-recording video to avoid re-recording the presentation.

Frequently asked questions

Does the video get re-encoded when audio is added?

No. ffmpeg uses -c:v copy to pass the video stream through unchanged. Only the audio is processed: your audio file is decoded and re-encoded to AAC, then muxed with the untouched video track. This makes the operation fast and keeps the original visual quality bit-for-bit identical.

Are my video and audio files sent to a server?

No. Both files are read into your browser's local memory and muxed there by ffmpeg.wasm. The resulting MP4 is downloaded directly to your device. Nothing is transmitted to Sunasty or any third party during the process.

What happens if the audio file is much longer than the video?

If you choose the stop-at-shorter option, ffmpeg adds -shortest and the output ends when the video finishes. If you choose loop audio, the audio track is repeated from the beginning until it reaches the video duration, then trimmed. In both cases the output length matches the video.

What is the difference between Replace and Mix mode?

Replace uses ffmpeg to drop the original audio stream entirely (-an on the input) and substitutes your chosen track. Mix uses the amix filter to combine the original audio and the new track into a single stream, letting you control how loud each one is relative to the other.

Which audio formats can I use as the added track?

ffmpeg.wasm decodes MP3, WAV, M4A, AAC, OGG (Vorbis), FLAC, and most common formats. The output audio is always AAC regardless of the input format, because AAC is the standard audio codec for MP4 containers and plays everywhere.

Why is the output always MP4 even if my video was a MOV or WebM?

MP4 with H.264 video and AAC audio is the universal playback format. When copying the video stream, the container is kept as MP4 to guarantee maximum compatibility. If you specifically need a WebM or MKV output, convert the result with the video-converter tool afterward.