The verb is pronounce, with the ow of now. The noun is pronunciation, with the u of cup, and its spelling has no o after the n. Both go back to the Latin pronuntiare, to announce, but English spells them differently, and each one is said the way it is written.
The same split shows up in other pairs: announce and annunciation, denounce and denunciation, renounce and renunciation. In every pair the verb has ow and the noun has the short u.
Saying pruh-nown-see-AY-shun is common, but Cambridge and Oxford list only pruh-nun-see-AY-shun, in both accents. Think of the word nun inside it, and put the main stress on AY.