# Sound

> Add music, effects and voice to a scene video: tracks on the timeline in seconds, fades, loops, ducking, credits, limits, and how the preview and the render mix them.

Source: https://video.geenius.io/docs/sound/

A scene video can have sound. It lives in the spec as a list of **tracks** on the video's own timeline.

## A track

| Field | Meaning |
|---|---|
| `kind` | music, sfx or voice |
| `source` | A sound you uploaded (`storage:<id>`) or an https address |
| `start`, `duration`, `offset` | Seconds on the timeline, and where in the file to start |
| `volume` | From 0 to 1 |
| `fadeIn`, `fadeOut` | Seconds |
| `loop` | Repeat to fill the duration |
| `duck` | The level music dips to under voice and effects |
| `credit` | Who made it, shown for your own records |

A video can have at most **24 tracks**.

## Preview and render

The preview plays the **same mix** as the picture. Browsers keep a page silent until it has been touched, so click or tap first. The browser render mixes all tracks to **one stereo bus** and writes **one AAC track** into the MP4. A video with no tracks has no audio track.

## Where sound comes from

- **Your own**: upload a file in the app, or use an https address.
- **A licensed library**: Video can search a library of licensed music and effects when the deployment has one connected. On a deployment without one, the search says so and you use your own sound.

> [!HONEST]
> Video does not generate music or speech.