This was one of those things im sure was changed last minute due to testing....it is very hard to judge "hearing" distance and what constitutes a loud or soft noise when playing a game...im also willing to bet that players could easily get through the whole section as long as they walked slow
EDIT: I stealthed by all except the first on my run through
Is it actually possible to avoid alerting some of them? Playing on hard it seemed pretty automatic that they responded