Not silently is permitted, by the sound of it (5)
I believe the answer is:
aloud
'not silently' is the definition.
(I've seen this in another clue)
'permitted by the sound of it' is the wordplay.
'permitted' becomes 'allowed' ('allow' can be a synonym of 'permit').
'by the sound of it' indicates a 'sounds like' (homophone) clue.
'allowed' is a homophone of 'ALOUD'.
'is' is the link.
(Other definitions for aloud that I've seen before include "that listeners can receive" , "So as to be heard" , "Audibly" , "Take part in, link" , "For all to hear" .)