Stealth aircraft are built to reduce their observability in 3 main areas, with the goal of reducing the warning time an enemy has:
- radar
- optical and IR
- sound
This is in order of detection range: Radar can find an aircraft potentially at hundreds of km, optical systems go to a few tens of km, and sound becomes a factor only when the aircraft is very near (at low altitude and high speed, you can get within 100 m of the target before you become audible).
The detection range also gives you the order of importance. Radar takes priority.
The sound produced by an aircraft is dominated by its engine noise. The air rushing over the skin makes some noise, but that's generally only audible when the engines are shut off. This means there's no benefit to reducing skin noise.
In addition:
a fringed, comblike leading-edge
This is a nightmare for radar stealth. You want the structure to be as simple and smooth as possible, because when you have a large number of surfaces in many directions, you get many radar returns. Improving audio stealth would compromise radar stealth.