Sound effects

Claude can't hear. So this skill never judges sound by feel: it measures every step, and it hands the user 5 timestamps to check with headphones. It never says "it sounds good".

The approach is Vox's sound-pass prompt (@Voxyz_ai, https://x.com/Voxyz_ai/status/2109358372873183564, 11 Oct 2026). Keep that credit.

Default kit (use this first)

sfx/manifest.json → default_kit: one sound per core category, picked by ear by Bart (12 Oct 2026). Use it unless the user asks for something else.

Everything else in the manifest is an alternative (alt_categories lists which categories can stand in for each other). See TOP-PICKS.md for credits.

Rules

• Never synthesize music or sound effects in code (no oscillator or sine beeps). Use real recorded files.
• Sound effects: use the bundled library first. sfx/manifest.json lists every file with its category, duration, source and licence. All bundled files are CC0.
• If the bundled library has nothing that fits, use the runtime entries in the manifest (Mixkit). Download them into the user's project, never into this skill folder, and never commit them. Mixkit allows use in videos but not standalone redistribution.
• Music: a track the user gives you, or one from MUSIC.md (suggested free tracks by feel, downloaded at runtime, never committed). Write the source and licence into SOUND.md. If the user has no track, suggest 2–3 from MUSIC.md that fit the video and let them pick.
• Licences: only CC0 and Mixkit Free License count as free and safe; the script checks every sound it places. Don't list licences sound by sound. If all are known, tell the user once: the sounds are listed as CC0 or Mixkit Free License by their sources, sources can change their terms so check the source page now and then. If the script reports an unknown licence, name that exact sound, its licence and its source link, and tell the user not to publish until it is checked or swapped. Never describe an unknown licence as probably fine.
• Music licence: ask where the track came from and pass --music-source "track, artist, link" and --music-license "...". Without them the run marks the music LICENCE NOT CONFIRMED; say so plainly.
• Credits: every run writes CREDITS.txt, a paste-ready list of the sounds actually used and the music. Recommend it once: optional, for a YouTube, Instagram or TikTok description; CC0 needs no credit, but the list is proof of source if a platform flags the audio.
• Measure the exported MP4, never the intermediate WAV.
• Measure the exported MP4, never the intermediate WAV.

0. Licence check (during the edit, before you export)

Sources change their terms, so check the current terms for what this video actually uses, every time, before the final export:

1. Use case. Ask where the video will be published, unless it is already clear (for example "my YouTube video"): YouTube, Instagram, TikTok, other social, online ads, a podcast, client work, TV or radio broadcast, a video game, CDs or DVDs, or a template or tool for other people. Pass it as --use-case "..." so it lands in SOUND.md.
2. Sources. For each source used in this video (the providers in CREDITS.txt, plus the music), open its current licence page (web fetch; if the site blocks it, the user's browser) and check two things: the terms still say what the table below says, and the use case is allowed. For Freesound, open the page of each sound actually used and confirm it still says Creative Commons 0. For the music, open the track's page.
3. Report it plainly, one line per source: "Checked today: Mixkit Stock Music Free License, unchanged, YouTube allowed." If a source changed its terms or the use case is not allowed, name the exact sound or track and why, and swap it before exporting. Never call an unchecked or unclear licence fine.

Mixkit forbids redistributing its files "in a tool or template", which is why this skill never stores Mixkit sounds or music: they are downloaded into the user's project at runtime.

1. Set up

Needs ffmpeg and Python 3.10+. Install into a folder you can delete:

python3 -m venv .sound-effects
.sound-effects/bin/pip install librosa soundfile numpy scipy matplotlib

2. Build the cue list

Read the video's duration and fps with ffprobe. List every moment that needs a sound. Take them from the composition source when you can (HyperFrames/Remotion timeline, GSAP labels, motion-broll plan.json). Otherwise detect scene cuts with ffmpeg -i in.mp4 -vf "select='gt(scene,0.3)',showinfo" -f null -. Write cues.json as [{"t": 2.0, "type": "scene_cut", "desc": "cut to card"}, {"t": 4.2, "type": "typing", "dur": 1.3, "desc": "..."}].

The cue types map to library categories (category_map in the manifest):

Use one sound per cue. Vary takes within a category so repeats don't sound robotic.

3. Beat grid, and check it

Run librosa.beat.beat_track. librosa sometimes locks onto the off-beat. Low-pass the music below 120 Hz, plot it against the beat lines for the first 4 bars, and compare where the kicks land. If the kicks don't sit on the lines, shift the whole grid by the median offset and plot again. Show the user beatgrid.png.

4. Put the big moves on the beat

Hard cuts and reveals should land on a beat. If you can change the animation timings, change them. If you can't, move where the music starts (search offsets and pick the one with the smallest cut-to-beat error). Report the error per cut in ms.

5. Place SFX and mix

Priority: voiceover > SFX > music. The music bed sits around -12 dB under the SFX and ducks under any voiceover by 8–12 dB. Target about -16 LUFS integrated with true peak below -4 dBFS, so nothing clips after AAC compression or on a phone speaker. Export, measure, adjust gain or add a limiter, then export and measure again:

ffmpeg -nostats -i after.mp4 -af ebur128=peak=true -f null - 2>&1 | tail -12
ffmpeg -nostats -i after.mp4 -ac 1 -af ebur128=peak=true -f null - 2>&1 | tail -12   # mono check

6. Prove it

For every cue, grab one frame just before and just after it from the exported MP4, and plot the waveform around it (checks/cue-XX.png). Then pick the 5 riskiest timestamps (largest beat error, long sounds such as typing and risers, the reveal) and give them to the user to check with headphones.

Original-quality files

The bundled sounds are compressed OGG Vorbis previews (trimmed, normalised, often mono), which is fine for most videos. If a sound is too low quality for the job (audible artefacts, a close-up hero sound, a final master), fetch the original:

• Look up the entry's source_url in sfx/manifest.json. Every bundled and default_kit entry has one.
• Freesound (freesound.org/people/<user>/sounds/<id>/): the original is the Download button on that page. It needs a free Freesound account, so ask the user to download it (or use their API key if they give you one).
• Kenney (kenney.nl/assets/<pack>): download the pack zip from that page. Each Kenney entry's original_file in the manifest names the exact file, e.g. kenney_interface-sounds.zip → Audio/click_001.ogg. The rule: drop the prefix (kenney-interface- → interface-sounds, kenney-ui- → ui-audio, kenney-digital- → digital-audio, kenney-impact- → impact-sounds, kenney-sci- → sci-fi-sounds), then match the rest inside Audio/ ignoring case, with - read as _ (kenney-digital-phaserup1 → Audio/phaserUp1.ogg, kenney-impact-impactpunch-heavy-000 → Audio/impactPunch_heavy_000.ogg). Kenney's originals are OGG too, but unprocessed.
• Save originals into the user's project (for example sound-effects-cache/originals/), never into this skill folder, and pass the file to the script via --swaps or by editing a copy of the manifest. Apply the same trim and level as the bundled file.

Find more sounds

When the user wants more or different sounds, search these sources, suggest 3–5 candidates with a link, creator and licence for each, and let the user pick. Before using any of them, open its page, confirm the licence shown today, and check it allows the user's use case (step 0); add it to CREDITS.txt. Only use or redistribute a sound as its licence allows. For music, see MUSIC.md.

Only CC0 sounds may be added to this repo's sfx/ folder, each with a manifest entry (file, category, duration, source_url, licence, attribution) and a row in LICENSES.md.

Script

scripts/sound_pass.py runs steps 3–6 end to end:

.sound-effects/bin/python scripts/sound_pass.py --video silent.mp4 --cues cues.json \
  --music track.mp3 --manifest sfx/manifest.json --out out/

Add --use-case "...", and --music-source/--music-license once confirmed. It outputs after.mp4, report.html, SOUND.md (licence check, log, cue → file table, LUFS and peak numbers, the 5 timestamps), CREDITS.txt, beatgrid.png and checks/. Read its log and fix anything over target. If you changed the animation timings, re-render first.

7. Swap report

sound_pass.py also writes report.html. It lists every cue with its timestamp, category, the sound used (with a play button), the source page, licence and credit, plus the alternatives in that category, each with a play button and a "let model decide" option. Open it for the user. They choose, click Download swaps.json, and you re-run with --swaps swaps.json. "auto" means pick another sound from the alternatives. Keep cue indices stable between runs.

Report to the user

Report the licence check (all known, or each unknown sound by name), the use case and whether it is allowed, the music source and licence (or that it is not confirmed), that CREDITS.txt is there if they want it, the beat-grid result (shifted or not, and by how much), the cut-to-beat errors, final LUFS and true peak (stereo and mono), and the 5 timestamps. Don't tell them it sounds good.