Baby Laughter is a last puzzle set in P.T..
In the last cycle, you need to trigger three baby giggles. The first thing you need to do after you step into the corridor is to wait until the clock turns to 00:00 (in the meantime you can either stand still or freely walk around the place). Once the bell begins ringing, you can trigger either of the two baby laughs below at any time and in any order.
Solve the Puzzle
Baby laugh 1 - Move your character ten full steps in any direction, and be sure to stop immediately on your 10th step. You can even walk up against a wall. This should trigger the first baby laugh.
Baby laugh 2 - Speak into your PlayStation microphone the letter "J" (or "Jay") and then stay completely silent for at least three full seconds (you do not have to be exceptionally loud or articulate when saying "J"). Continue to say "J" into the mic between at least three full-second intervals of silence. After saying "J" a total of three, four, or even five times (just keep saying it), this should trigger the second baby laugh.
Baby laugh 3 - After you get the second laugh, your controller should vibrate for a few seconds. Stand still and do absolutely nothing and you should get the third and final giggle. The telephone starts ringing. Approach it, zoom in and you'll get the message "You've been chosen".
After solving this final puzzle, the door is unlocked.