Halloween narrator voiceover with spooky music for a video

To score a Halloween story video, generate the narration with Sume text-to-speech, generate an instrumental bed with the music router, and mix them in one Timeline 1.0 render with the bed ducked under the voice. Three calls, with two prices you can add up: $0.0475 per 1,000 characters of narration and $0.125 per accepted music generation.

Step 1: the narration

Write 400 to 800 characters per scene so a take stays short. Set language explicitly, since it defaults to English if omitted. Slow the voice down with generation_config.speed (range 0.6 to 1.5) and add a mood with generation_config.emotion, a free text field up to 64 characters, for example "hushed, ominous". Whether a given voice follows that hint closely is something to listen for, not assume.

Step 2: the music bed

The music router prompt takes 1 to 5,000 characters. There is no duration field, so say the length in the prompt. A prompt built from the seven axes in the music docs works well: emotion, genre, BPM, key, instruments with texture, an arc, and era. Close with "Instrumental, no vocals."

Foreboding, slow-burn cinematic horror. 62 BPM, D minor.
Bowed cello drone, detuned music box, distant low piano,
sparse reverse cymbal swells. A 2-minute track.
Stays quiet, then one sudden string stab at 1:30.
Analog 1970s horror score production. Instrumental, no vocals.

Step 3: mix it

Pass the narration as the audio spine and the music as soundtrack in Timeline 1.0. Set soundtrack.gain_db, duck_db (0 to 20) to lower the bed while the voice speaks, fade_out_seconds (up to 10) so the music does not stop dead, and loop if the bed is shorter than the story. Ducking needs a real voice spine; it is rejected with a silent track. Render is $0.10 per output minute, and POST /v1/timeline-1.0/plan is unbilled, so check the plan first. Details: Timeline 1.0.

Limits

The music router rejects a non-empty negative_prompt, so describe what you want rather than what to avoid. Generated music is not guaranteed to hit your timestamp, so listen to the stab before you publish. See looping a bed under a long video.