The uses of diacritic marks in modern English are quite limited, and diacritic marks can always be omitted without being incorrect. This applies equally to A and any other letter.
Recent foreign borrowings: e.g. rôle, coup d'état, façade, etc. but role, coup d'etat, and facade are also all correct.
Stage and poetry prosody: e.g. learnèd indicating a normally silent vowel is to be pronounced
Diaeresis: e.g. Brontë, Chloë, Zoë (also indicating a silent vowel is to be pronounced) or naïve, Boötes, and noël (indicating a vowel is to be pronounced separately).
Artistic or humorous effect: e.g. Blue Öyster Cult
As for pronunciation: in English, including or not including the diacritic does not change the pronunciation, though it may clarify it. In other languages diacritics have many different pronunciations and you would have to look at each language for details.