AI background music for a video: Lyria 3.5 plus Timeline soundtrack

To add AI background music to a video, generate an instrumental bed with the Sume Music Router (Lyria 3.5) and set it as the Timeline soundtrack with duck_db so it drops under the voice. Google's Gemini post lists background music as a Lyria 3.5 template, read 2026-10-01.

How do I make the bed?

End the prompt with "Instrumental, no vocals." Keep arrangement sparse so speech stays clear. It costs a fixed $0.125.

How do I mix it under narration?

Import the track to media.sume.com, then pass it as soundtrack. Timeline renders at $0.10 per output minute.

"soundtrack": {
  "url": "https://media.sume.com/artifacts/artf_demo/bed.mp3",
  "gain_db": -8,
  "duck_db": 10,
  "loop": true,
  "fade_out_seconds": 4
}

What does each step cost?

What are the limits?

Ducking needs a real audio spine, not silence. There is no EQ or compressor, so pick a bed that is already balanced.