a M.A. degree, or an M.A. degree

For abreviations, as opposed to acronyms, do you use “an” before a vowel phoneme?

Pronunciation rules, so it doesn’t matter if it’s an abbreviation or an acronym.

“An M.A. degree” is usually used since it’s pronounced “An em-ay degree.” If it were pronounce “ma,” then you’d use “a.”

That is correct. It doesn’t matter how it is spelled. It only matters if it starts with a vowl sound.