Sound of tranquillity in musical work (5)
I believe the answer is:
piece
'musical work' is the definition.
(I know that musical work can be written as piece)
'sound of tranquillity' is the wordplay.
'sound of' indicates a 'sounds like' (homophone) clue.
'tranquillity' becomes 'peace' (peace is a kind of tranquillity).
'peace' is a homophone of 'PIECE'.
'in' is the link.
(Other definitions for piece that I've seen before include "Queen on board, perhaps" , "Section; firearm" , "wedge a > bit" , "man on board" , "Bit - part" .)