UtilityKit

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Add Audio to Video

Add or replace the audio track on any MP4, WebM or MOV video — keep, mix, or replace the original audio.

About Add Audio to Video

Add Audio to Video pairs any audio file (MP3, WAV, AAC, OGG, M4A) with any video file (MP4, WebM, MOV). You can either fully replace the original audio or mix the new audio over the existing track at equal level. The video stream is copied losslessly — only audio is encoded to AAC at 192 kbps for broad compatibility. A 'shortest' length policy trims the output to whichever stream ends first, useful when adding a soundtrack to a clip that should end at the music's natural finish. Everything runs in your browser via ffmpeg.wasm — neither the video nor the audio ever uploads to any server.

Why use Add Audio to Video

  • 100% private — neither video nor audio ever leaves your device
  • Lossless video stream-copy — no quality reduction in the picture
  • Choose to replace or mix with the original audio
  • AAC 192 kbps audio for broad playback compatibility
  • No watermarks, no signup, completely free
  • Works offline after the first ffmpeg.wasm cache

How to use Add Audio to Video

  1. Drop your video file (MP4, WebM, MOV) into the first upload area.
  2. Drop your audio file (MP3, WAV, AAC, OGG, M4A) into the second upload area.
  3. Pick a length policy: 'Match video length' trims either side to the video's duration; 'Keep full audio' lets the output extend until the audio finishes.
  4. Tick 'Keep original audio mixed in' if you want the existing soundtrack to stay (mixed at equal volume); leave unticked to fully replace it.
  5. Click 'Combine Video + Audio' and wait for ffmpeg.wasm to mux locally.
  6. Preview the combined result with sound and click Download.

When to use Add Audio to Video

  • Adding a music soundtrack to a silent or muted screen recording
  • Replacing copyrighted music with royalty-free audio before posting
  • Adding voiceover narration to a slideshow or product demo
  • Mixing a soft background track over an existing dialogue video
  • Pairing a dance / gameplay clip with a custom audio track
  • Adding ambient sound to silent drone or scenic footage

Examples

Add music to a silent screen recording

Input: 200 MB silent MP4 + 5 MB MP3 song, replace mode

Output: ~205 MB MP4 with new soundtrack, ends at video length

Mix narration over dialogue

Input: 100 MB MP4 with dialogue + 8 MB MP3 narration, mix mode

Output: ~104 MB MP4 with both audios mixed equally

Pair drone footage with audio

Input: 150 MB MP4 (no audio) + 10 MB ambient WAV, replace mode

Output: ~152 MB MP4 with ambient soundtrack

Tips

  • If your new audio is much shorter than the video, use 'Keep full audio' so the output ends naturally with the music.
  • When mixing both tracks, normalize the new audio to roughly the same loudness as the original beforehand for the best balance.
  • For copyright-safe music, pair this with royalty-free libraries like the YouTube Audio Library or Freesound.
  • Use Video Mute first if you want to start from a fully silent base before adding new audio.
  • Tagging the audio file's metadata (title, artist) before combining preserves it through the AAC re-encode in some players.
  • After the first run ffmpeg.wasm is cached so subsequent combines start instantly even offline.

Frequently Asked Questions

Does my video leave my device?▾
No. Both files stay on your device — combination runs 100% locally via ffmpeg.wasm with no upload to any server.
Why does it require Chrome / Edge?▾
ffmpeg.wasm requires SharedArrayBuffer, which is gated behind COOP/COEP cross-origin isolation. Chrome, Edge, Brave, and Arc all qualify.
What's the file size limit?▾
No server limit. Browser memory caps the combined practical limit around 500 MB across both files.
Will combining reduce video quality?▾
No. The video stream is copied unchanged. Only the audio is re-encoded to AAC 192 kbps.
What does 'Keep original audio mixed in' do?▾
When ticked, the original video's audio is mixed with the new audio at equal volume using FFmpeg's amix filter. When unticked, the original audio is replaced entirely by the new track.
What if the audio is longer / shorter than the video?▾
'Match video length' trims either to the video's duration (audio cuts off if longer; output ends early if shorter). 'Keep full audio' lets the output extend to whichever stream ends last.
Does this work with WebM / Opus audio?▾
Input WebM and Opus audio are accepted, but the output is always MP4 with AAC for compatibility. Use Video Resize & Converter if you need a different output container.
Why does the first run take longer?▾
ffmpeg.wasm (~30 MB) downloads on first use and is then cached, so subsequent combines start instantly even offline.

Explore the category

Video Mute / Audio Remover Strip the audio track from any MP4, WebM or MOV video, lossless and instant. Video to MP3 Extractor Extract and save the audio track from any MP4, MOV or WebM video as MP3, WAV or OGG. Fully browser-based. Video Trimmer Cut and trim MP4, WebM, MOV videos to exact start and end times, right in your browser. Video Compressor Reduce MP4, WebM and MOV file size with adjustable H.264 CRF compression — runs entirely in your browser. Video Merger Concatenate multiple MP4, WebM or MOV clips into one continuous video file — no upload required. Video Cropper Crop a rectangular region out of any MP4, WebM or MOV using a draggable selection rectangle.
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Glossary

amix filter
FFmpeg audio filter that mixes multiple input streams into one. amix=inputs=2 mixes two tracks at equal level.
-shortest
FFmpeg flag that ends the output at the shortest input stream — useful for trimming audio that exceeds video duration.
AAC
Advanced Audio Coding — the audio codec inside MP4 with broad device support and good quality at moderate bitrates.
Mux
Combining (multiplexing) separate video and audio streams into one container file like MP4.
ffmpeg.wasm
WebAssembly port of FFmpeg that runs locally in your browser tab.
SharedArrayBuffer
JS memory primitive needed for multi-threaded WebAssembly; gated by COOP/COEP.