I have heard it pronounced at least three ways:
1) Muh-MY-uh (rhymes with Papaya)
2) Muh-ME-uh (like the Italian phrase "Momma-miya!" without the middle "ma"
3) MAH-mmy-Uh (like Mammy-uh, with the accent on the first syllable)
I recently got an RB-67, and when I'm out shooting with it, I sometimes get asked what kind of camera it is. I'd like to know how to say it right. I have started just calling it an RB67.