Answer:

shh

Answer	Crossword Clue
SHH	"Can it!"
SHH	"Put a sock on it!"
SHH	"Keep it down"

SHH	Quieting sound
SHH	"Pipe down or they'll find us!"
SHH	Response to a text alert in the movie theater
SHH	Command in a library
SHH	Librarian's admonition
SHH	"Keep it down!"
SHH	"Pipe down!"
SHH	Proctor's warning

SHH	Theater admonition
SHH	"No talking!"
SHH	Silencer
SHH	Theater warning
SHH	"Not a peep!"
SHH	"Quiet, please!"
SHH	Auditorium admonition
SHH	Librarian's reproof

SHH	"Use your inside voice!"
SHH	Whisperer's comeuppance
SHH	Reaction to a big library volume?
SHH	Finger-on-lips sound
SHH	Library admonition
SHH	"Quiet!"
SHH	"Zip it!"
SHH	Reaction to a library volume?

SHH	Finger-to-lips syllable
SHH	Recital rebuke
SHH	Librarian's reminder
SHH	Volume control?
SHH	"Zip your lip!"
SHH	Classroom caution
SHH	Finger-to-lips sound
SHH	Librarian's admonition, maybe

SHH	"No whispering!"
SHH	"Put a lid on it!"
SHH	"Put a sock in it!"
SHH	"Stifle it!"
SHH	"Quiet down!"