Observes musical sounds (5)
I believe the answer is:
notes
I believe this is a double definition.
'observes' is the first definition.
('note' can be a synonym of 'observe')
'musical sounds' is the second definition.
(Other definitions for notes that I've seen before include "Takes into account" , "Short written messages" , "eg E F G (mus.)" , "Stone with jottings" , "Observes; short comments" .)