Even ebb and flow in sound (4)
I believe the answer is:
tied
'even' is the definition.
('tied' can be a synonym of 'even')
'ebb and flow in sound' is the wordplay.
'ebb and flow' becomes 'tide' (I've seen this before).
'in sound' indicates a 'sounds like' (homophone) clue.
'tide' sounds like 'TIED'.
(Other definitions for tied that I've seen before include "have equal scores" , "United" , "Roped into diet" , "Linked; restricted" , "(Of pub) owned and run by a brewery" .)