Buzzer with quiet sound (4)
I believe the answer is:
beep
'sound' is the definition.
(beep is a kind of sound)
'buzzer with quiet' is the wordplay.
'buzzer' becomes 'bee' (I've seen this before).
'with' says to put letters next to each other.
'quiet' becomes 'p' (music symbol for 'piano').
'bee'+'p'='BEEP'
(Other definitions for beep that I've seen before include "Sound of a horn" , "High-pitched noise" , "Toot" , "Alarming noise" , "Electronic noise" .)