> ## Documentation Index
> Fetch the complete documentation index at: https://vidow.io/docs/llms.txt
> Use this file to discover all available pages before exploring further.

# Add Audio to Video — Replace or Overlay Audio Tracks

> Add background music, replace the audio track, or overlay narration on any video file. Browser-based processing, no uploads required.

<Info>Add audio is a **Professional** feature. [Upgrade your plan](/billing/plans) to access it.</Info>

The Video Downloader Plus add audio tool lets you replace a video's existing audio track or mix in a new one — background music, narration, or any audio file. All processing runs locally in your browser.

## How to add audio to a video

<Steps>
  <Step title="Open the tool">
    Go to [vidow.io/tools/add-audio](https://vidow.io/tools/add-audio).
  </Step>

  <Step title="Upload your video">
    Click **Upload video** or drag a video file onto the page.
  </Step>

  <Step title="Upload your audio file">
    Click **Upload audio** and select the audio file you want to add.
  </Step>

  <Step title="Choose a mode">
    Select **Replace** to swap out the existing audio track, or **Overlay** to mix the new audio with the original.
  </Step>

  <Step title="Adjust timing">
    Set the offset for when the audio should start relative to the beginning of the video.
  </Step>

  <Step title="Export">
    Click **Export**. The output file is saved to your downloads folder.
  </Step>
</Steps>

## Supported formats

|                 | Formats                  |
| --------------- | ------------------------ |
| **Video input** | MP4, WebM, MKV, MOV      |
| **Audio input** | MP3, AAC, M4A, WAV, FLAC |
| **Output**      | MP4, WebM                |

## Audio modes

| Mode        | Description                                                                   |
| ----------- | ----------------------------------------------------------------------------- |
| **Replace** | Removes the original audio track and replaces it with the uploaded audio file |
| **Overlay** | Mixes the new audio with the existing audio track at equal volume             |

## How it works

Audio processing runs locally in your browser using a WebAssembly build of FFmpeg. The tool muxes your audio file into the video container without re-encoding the video stream, keeping processing fast and preserving video quality. No data is uploaded to any server.

## Related tools

<CardGroup cols={3}>
  <Card title="Merge Videos" href="/tools/merge-videos">
    Join multiple clips before adding audio.
  </Card>

  <Card title="Video Trimmer" href="/tools/video-trimmer">
    Trim your video to the right length first.
  </Card>

  <Card title="Video Compressor" href="/tools/video-compressor">
    Reduce file size after adding the audio track.
  </Card>
</CardGroup>