One inheriting man’s sound sort of post (7)
I believe the answer is:
airmail
This might be a double definition.
'one inheriting man's sound' is the first definition.
The definition and answer can be both related to communication as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'sort of post' is the second definition.
(Other definitions for airmail that I've seen before include "Letters that are flown in" , "Flying post?" , "Postal system f" , "Letters sent up" , "Speedy deliveries" .)