Man’s sound? It must be the armour (4)
I believe the answer is:
'the armour' is the definition.
(mail is a kind of armour)
'man's sound?' is the wordplay.
'man' becomes 'male'.
'sound?' indicates a 'sounds like' (homophone) clue (I've seen 'sounds' mean this).
'male' is a homophone of 'MAIL'.
'it must be' is the link.
(Other definitions for mail that I've seen before include "some self-defence material" , "Post or body armour" , "American post from Lima" , "Eg, letters" , "letter?" .)