Discover to be sound? (6)
I believe the answer is:
rumble
This clue is a double definition.
'discover' is the first definition.
(to rumble someone is to discover their secret)
'be sound?' is the second definition.
(I have seen 'Sound of thunder' mean 'rumble' so perhaps 'sound' could also mean 'rumble')
'to' acts as a link.
(Other definitions for rumble that I've seen before include "Utter in deep voice, of have a slang fight - in the jungle?" , "Harlem gangs might" , "Sound of thunder or a hungry stomach - discover (plans)" , "be wise to" , "Low, vibrating (traffic) noise" .)