Yeah, spoken language takes precedent over written in A/An rules. Especially in the case of abbreviations, where the abbreviation BECOMES the word, regardless of what it abbreviates. "An MMOG" is correct, in this case. Same reason you'd say "AN FBI badge," even though "A federal bureau of investigation badge" is being abbreviated.
This is also why it's generally considered correct for a British person to write and say "An hospital," and incorrect for an American to do so. They (again, generally) don't pronounce the hard H, while we do.
English lesson complete!
This comment was edited on Sep 17, 17:28.