Synonyms for Quieten:

n.

block out, sounds, deaden, cut out, muffle, dull, drown, shush. quieten (noun)
hush up, calm down, quiesce, tranquillize, still, silence, shut up, quiet, hush, quiet down, lull, pipe down, tranquillise, calm, tranquilize.

v.

calm (verb)
tranquilize.
deaden (verb)
drown.
hush (verb)
silence.
quiet (verb)
calm down.
quieten (verb)
lull, hush, pipe down, quiet, deaden, muffle, quiet down, dull, shut up, still, shush.

Other synonyms:

shush. deaden, muffle. drown. dull. silence
quiet down, shush.
Other relevant words:
dull, shush, deaden, muffle.