At high wing angles, why can the comb-like feathers on an owl's leading edge make airflow quieter?

Answer: They split rough airflow into small swirls that are quieter.

They sharpen the wing edge so air slides past cleanly. — A sharp edge sounds plausible because a knife cuts cleanly through air. Owl combs are not blades, and the problem at steep angles is air changing suddenly over the upper surface. That change forms noisy, draggy structures. A sharper edge would not disturb the flow early enough to soften that breakup.

They split rough airflow into small swirls that are quieter. ✓ — The combs sit where air first meets the leading edge. At high angles, that air can break into rough, noisy structures; the combs split it early into smaller swirls. The later breakup is less severe, so the flow is quieter. This is passive flow control, not soundproofing.

They trap a cushion of air under the feathers. — Air trapped under feathers could seem to muffle sound like insulation. The front comb is on the leading edge, where incoming air first meets the wing, so its effect depends on flow around that edge. A cushion under the feathers would not explain why the effect changes with wing angle. The useful idea is timing: nudge the flow before the loud breakup forms.