Observe sound location (4)
I believe the answer is:
site
'location' is the definition.
(site is a kind of location)
'observe sound' is the wordplay.
'observe' becomes 'sight' (I've seen this before).
'sound' indicates a 'sounds like' (homophone) clue.
'sight' is a homophone of 'SITE'.
(Other definitions for site that I've seen before include "Building land" , "Locality" , "Location (of building, dig etc)" , "setting" , "Where something is located" .)