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soundtrack

This skill should be used when the user asks to "add music to the animation", "make the music in code", "synthesize a soundtrack", "compose background music that fits the cuts", "sync the music to the video", "check if the audio lines up with the picture", or "the music is too sudden / startling". Covers a code-synthesized score that lives inside the animation page (Web Audio API, rendered offline to WAV), AI-generated or user-supplied music, and an automatic check that cuts land on the beat without a startling jump in loudness.

skills/soundtrack/SKILL.md

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Sep 30, 2026
License
MIT

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# Soundtrack

## Write the sound for this film

A film's sound is written for that film: its own instruments, rhythm and harmony in Web Audio, from nothing, in `window.SCORE = ac => { … }` (oscillators, noise, filters, envelopes, delays: whatever the piece calls for). Drums, a groove or a music-box bed are written the same way, for this film: no two films should sound alike because they share a template. `templates/groove.js` and `templates/score.js` are two ready-made scores, used only when the user asks for them; each file documents its options.

`render.mjs` (from the `javascript-animation` skill) renders `window.SCORE` offline with `OfflineAudioContext`, writes `<out>.wav` and muxes it into the MP4; the page's live preview plays the same score when clicked.

Don't ask the user to choose a music source up front. At delivery, mention in one line that AI-generated music or their own track also work.

## How the music follows the picture

- **Tempo map.** Each scene gets a whole number of bars (the tempo may flex per scene), so every cut lands on a downbeat.
- **Mark cuts with harmony, not volume.** Each scene walks home to the dominant and the cut arrives on the tonic. A loud accent on a cut (bass + bright chime at once) reads as a jump scare: users flagged a +7 to +10 dB jump as startling; +3 to +5 dB with a chord change reads as a clean edit.