Was familiar with the latest sound (4)
I believe the answer is:
knew
'was familiar with' is the definition.
(I've seen this before)
'the latest sound' is the wordplay.
'the latest' becomes 'new'.
'sound' shows a homophone (sound like).
'new' sounds like 'KNEW'.
(Other definitions for knew that I've seen before include "Was familiar with" , "Acknowledged" , "Was cognisant or aware of" , "Was cognizant of" , "Recognised" .)