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Open Cast

Mixing

Why does the music get quieter when someone talks?

That is auto-ducking. The music drops under the voice and comes back in the gaps, automatically — and because the duck follows your actual voice clips, it stays correct after you cut. Trim a minute of filler and the music re-balances itself. You never draw a volume envelope by hand. You can set how far it drops, or switch it off.

Where
Main Editor → select the music lane → inspector.
The music level drops while the voice is speaking and rises back in the gap between sentences. The curve follows your voice clips, so it stays correct after you cut — you never draw it by hand. In the Main Editor the control sits on the music lane, shown as an Auto-ducking chip with its depth in dB.
What is happening under the hood. The control itself is the Auto-ducking chip on the music lane.

That is the part other tools leave to you: in an editor where the music is a track you imported, every edit to the speech means going back and redrawing the automation around it.

If the music still feels loud under the voice, deepen the ducking and lower the music level — not the master, which just makes the whole episode quieter.