Level of drop in sound (4)
I believe the answer is:
tier
'level' is the definition.
(I know that level can be written as tier)
'drop in sound' is the wordplay.
'drop' becomes 'tear' (teardrop).
'in sound' shows a homophone (sound like).
'tear' sounds like 'TIER'.
'of' acts as a link.
(Other definitions for tier that I've seen before include "Rank - row" , "Layer, rank" , "Level - layer" , "Row (of seats)" , "Rank, level" .)