Visitor's supposed to be heard (5)
I believe the answer is:
guest
'visitor's' is the definition.
(someone visiting a place)
'supposed to be heard' is the wordplay.
'supposed' becomes 'guessed' ('guess' can be a synonym of 'suppose').
'to be heard' shows a homophone (sound like).
'guessed' is a homophone of 'GUEST'.
(Other definitions for guest that I've seen before include "Visitant" , "Invited visitor" , "Someone invited" , "One who's invited" , "Visitor, lodger" .)