Besides ordinary airflow noise, what sound can the velvety surface on owl flight feathers help reduce?

Answer: Feathers rubbing against each other

Feathers rubbing against each other ✓ — The velvety layer works like a soft, dry buffer between neighboring feathers. Experiments that rubbed feathers together found velvet-covered feathers made much less sound, so the point is not only how air flows over the wing but also how the wing's own parts slide against each other.

Large swirls behind the wingtip — Wingtip swirls can make aerodynamic noise, and owl wings have other features that affect airflow. The velvet result is narrower: it points to quieting the scratchy friction sound made when feathers slide over feathers.

Muscle signals inside the wing — Muscles create movement, but their electrical signals are not the flight noise prey would hear. The measurable sound here comes from physical contact between feather surfaces, a bit like making a rough fabric quieter when it rubs.