In Monsters University, during the Scare Final, Dean Hardscrabble tells Sullivan that roaring at a child for whom it is not the appropriate scare...
...wouldn't make him scream, it would make him cry, alerting his parents, exposing the monster world, destroying life as we know it
Although the second half sounds a bit far-fetched (perhaps an attempt by Hardscrabble to really drive the point home to Sullivan), I'm left wondering about the first half. If a child's crying would alert the parents, why not a scream?
The very beginning of the film shows that it doesn't take much for parents to come to check up on the child ("I thought I heard something.") and the Monsters don't appear to bring any equipment with them into the Human world (definitely nothing like the scream extractor Randall builds in Monsters, Inc.). Indeed, they seem careful to leave any Monster artefacts behind before entering a door (as Frank McCay does with his MU cap). So I was thinking that perhaps the child's room itself is somehow modified to absorb specific sound waves and transmit them into the scream canister attached to the door station? That might explain why the first time a door is opened, it's done in an isolated, darkened lab environment (as can be seen during the orientation tour Mike attends on his first day at MU).
Do the Monsters have some way to completely extract the scream the child produces, preventing it from reaching the parents' ears?