Observe a sound (4)
I believe the answer is:
note
This clue is a double definition.
'observe' is the first definition.
(synonyms)
'a sound' is the second definition.
(thesaurus)
(Other definitions for note that I've seen before include "Pay attention to; record" , "Record; memo" , "held by piano teacher" , "Make a record of" , "register" .)