A sound, spoken declaration (6)
I believe the answer is:
avowal
'declaration' is the definition.
(avowal is a kind of declaration)
I cannot understand how the rest of the clue works.
Can you help me to learn more?
(Other definitions for avowal that I've seen before include "Frank confession" , "Open assertion" , "Open declaration or admission" , "Declaration (of truth)" , "Assertive declaration" .)