Observe sound (4)
I believe the answer is:
note
This clue is a double definition.
'observe' is the first definition.
(I know that observe can be written as note)
'sound' is the second definition.
(thesaurus)
(Other definitions for note that I've seen before include "Quaver, say" , "Mark - sound - money" , "Money-carrier > hit repeatedly by 'charmingly sweet' singer" , "Fame - memorandum" , "Tone of a short record" .)