Summit goes into matter, turning sound off (5,2)
I believe the answer is:
speak up
'sound off' is the definition.
(both can mean to give an opinion)
'summit goes into matter turning' is the wordplay.
'summit goes' becomes 'peak' (I can't explain this - if you can you should give a lot more credence to this answer).
'into' means one lot of letters goes inside another.
'matter' becomes 'pus' (pus is a kind of matter).
'turning' shows that the letters should be reversed in order.
'pus' back-to-front is 'sup'.
'peak' going inside 'sup' is 'SPEAK UP'.
(Other definitions for speak up that I've seen before include "Talk louder, can't hear you" , "augment volume" , "Raise one's voice" , "Raise your voice - can't hear you" , "Make oneself heard" .)