Bound sound? (5)
I believe the answer is:
rhyme
'bound sound?' is the definition.
The answer and definition can be both related to communication as well as being singular nouns.
Perhaps you can see an association between them that I can't see?
This is all the clue.
(Other definitions for rhyme that I've seen before include "Sound correspondence" , "Bit of poetry" , "Poem, eg for nursery" , "something read" , "Simple verse" .)