It's not turning the key that makes the sound (4)
I believe the answer is:
tone
'the sound' is the definition.
(tone is a kind of sound)
'it's not turning the key' is the wordplay.
'it' becomes ''t' (abbreviation. e.g. in 'tis).
'not' becomes 'no' (negative).
'turning' shows that the letters should be reversed in order.
'the key' becomes 'E' (a musical key).
'no' backwards is 'on'.
't'+'on'+'e'='TONE'
'that makes' acts as a link.
(Other definitions for tone that I've seen before include "Colour quality; note" , "feel" , "Quality of sound, etc" , "Pitch of a voice" , "Sound signal on a telephone" .)