Observe a 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)
'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" .)