Dearest creature in creation, studying English pronunciation...
The spelling can only be a guide to pronunciation, so ultimately you need to learn how to spell and pronounce each word.
In the case of "pro-nun-ci-a-tion," The stress falls on "nun", as "pro" is unstressed, the vowel is reduced to a schwa.
In some words, like "combat" there is a change of stress depending on the function of word. As a noun, the first syllable is stressed and so gets the [ɑ] pronunciation. As a verb the stress moves to the last syllable, and so the first vowel gets reduced. There are plenty of words that have this kind of moving stress, but there used to be even more. It is likely that in the future this feature of English will be lost (but you might have to wait a hundred years). There quite a range of pronunciations of "combat". In some dialects the pronunciation of the verb and noun are now unified. In others they are distinct.
/ verb kəmˈbæt, ˈkɒm bæt, ˈkʌm-; noun ˈkɒm bæt, ˈkʌm- /
In "format" the vowel sound is influenced by the "r", vowels which are followed by "r" usually have different pronunciation: compare "e" with "er" or "a" with "ar". (Format is word in which the pronunciation of the verb and the noun have been unified, there used to be a stress change, but there isn't anymore)
When learning new words, make a note of the stress, and remember that there is a lot of variation in vowel sounds between dialects, and so English speakers are tolerant of a range of vowel pronunciations.