If I use an abbreviation like MMU ("memory management unit" in computers), should I write "A MMU" or "An MMU"? Is it the pronunciation or the letter that decides?
The thing that matters is whether or not the proceeding sound is a vowel sound. There's actually a very similar example on the Grammar Girl site, A Versus An (emphasis mine):
The letters o and m can be tricky too. Usually you put an before words that start with o, but sometimes you use a. For example, you would use a if you were to say, “She has a one-track mind,” because one-track starts with a w sound. Similarly, “She has an MBA, but chooses to work as a missionary.”
Assuming you pronounce it em-em-yoo (initialism), then I would use "an MMU".