Have you ever noticed how different the first and second stanzas sound? Try reading the poem aloud – you can’t fail to notice it. The first stanza sounds super-confident because of all those declarative statements.
We think there are two ways to read the first stanza. The first is to read it like you would read a grocery list: completely matter-of-fact. "Things fall apart; the centre cannot hold; Mere anarchy is loosed upon the world" could sound like, "Three dozen eggs; a quarter-pound of cheddar cheese; Four packages of hickory-smoked bacon." The first stanza is basically a list of everything that has gone wrong with the world, but the sentence clauses are so short that they create a lot of regularly-spaced pauses: hence, the grocery list. This would be an ironic reading.
The second possibility is to read it as if you were one of those end-of-the-world types who stand on street corners shouting about one conspiracy or another and grabbing pedestrians by the collar as if to say, "Don’t you GET IT!" This way of reading would really take advantage of Yeats’s stressed beats: "TURNING and TURNING in the WIDE-ning GYRE."
The second stanza, we think, sounds like a Shakespearean soliloquy. Shakespeare often liked to have his characters project their private doubts and fears in the form of a speech, resulting in a lot of emotional ups and downs. Here, too, the speaker of Yeats’s poem seems to be going through a lot of mood swings. First, he sounds confident with his use of the word "surely," although we’re not so convinced when he uses it again in the next line. Then, he veers off into a suspenseful portrait of the sphinx, which we would read in a low voice, as if you were narrating a weird dream you had.
Finally, we think the last question shouldn’t really be read as a question, but more like another declarative statement. The speaker seems to have figured something out – he’s got his former confidence back. He has come to some definite conclusions regarding this whole world-in-crisis thing, and now, with this summary statement, he’s going to rejoin the rest of the characters in the play (whoever they might be – use your imagination!).