Nothing protects against line of sight / disturbance to environment. Skill 100 sneak, wearing troll armor, if a mob has line of sight, it sees you.

As for the "silence" of the huntsman's bow I only know what I have read and/or seen, i have honestly not done any testing trials.

Guessing if an arrow hits a tree or rock it'l make a sound outside the "protection" of any given bow stats. But I dunno, maybe i'll look.