I've always prononced it like "axio" 或者 "atsio", because if 你 read it in spanish (I'm mexican) it sounds like that. But I know since the first movie that is "akkio", but I can't say it like that...
I would say ack-see-oh. Because there are two c's 下一个 to each other, I just thought it would be like "access" The first C is like a K and the second, being ruled out 由 the first, becomes an S sound. I know it seems complicated, but that is grammatically correct. Of course, sometimes I say ah-see-oh from the audiobooks.