Sounding hard, nothing has replaced one chapter in something easy (6)
I believe the answer is:
phonic
'sounding' is the definition.
(relating to the sounds of a language)
I don't understand the remainder of the clue.
(Other definitions for phonic that I've seen before include "Chopin (anag) - related to learning by sounds" , "Of vocal sounds" , "Chopin (anag) - relating to sound" , "Involved with sound" .)