I don't know how to feel about this. Part of me thinks most zombies should be unintentionally making a modest amount of noise just by walking around without attempting to be quiet. And, if they cross a noise block (trash, glass, etc.) it should make the same noise players make when crossing them.
I think the frequency and volume of zombie vocalizations should be random within some range, like their health. I think the occasional quiet zombie is a nice change of pace.
If they were looking for a new type of zombie I think a silent one (no walking noises, no vocalizations) would be kind of cool, specially if they could look like a pile of trash or something when prone.
I typically play stealth, so headphones are on when in POIs. Maybe that helps?
Boars? They don't seem sneaky. I guess I don't really know what to expect from a boar. Mountain Lions and Wolves though could be sneaky. I think Mountain Lions hiding under the snow covered trees would be both cool and disturbing, worthy of the jump scare that would result.