This poem is a steal! You get way more bang for your poetry-buck with this one. Why, you ask? Well, you get three speakers for the price of one. Tough to beat a deal like that.
As detailed in the "Summary," this poem has three distinct voices. There's the poet-speaker that is observing the scene as he takes his evening stroll, there is the lover-speaker who is overheard by the poet-speaker singing to his beloved under the bridge, and there is the clock-speaker who warns us "you cannot conquer Time." Okay, the gang's all here.
Poet
Even though we are calling the first speaker the poet-speaker, it's important to remember that this isn't necessarily Auden himself. Auden often employed different voices and characters as the "I" in his poems. This speaker seems like he might take this walk on a regular basis. He knows the street names and he seems pretty at home. The tone of his lines is fairly even, flat and observational: "Walking down Bristol Street." It doesn't get much more reporterly or direct than that. Not much embellishment with this guy. Even the description of the river as "brimming" feels more like a tidal report than a flowery line of poetry. He doesn't get too emotional, even when the clocks start talking (Come to think of it, perhaps he should have been a little more freaked out by that).
Lover
The lover-speaker, on the other hand. is pretty over-the-top. This guys got the love-bug bad, and there isn't much anyone can do about it. He says some pretty dramatic stuff, but it seems like he really believes what he's saying. The lines sound pretty earnest. He also sounds a little more casual than our other speakers. For example, he uses "till" instead of until: "I'll love you / Till China and Africa meet." That "till" makes the lover-speaker's lines sound more honest and unrehearsed.
Clock
The clock-speaker sounds more desperate and formal than the other two speakers. He keeps starting his lines with those formal sounding, pleading O's ("O plunge your hands in the water"), and he keeps repeating words giving the sense that he's begging the reader to act ("O look, look in the mirror" and "O stand, stand at the window").
So, three speakers and each one adds their own flavor to the poem. See, we told you this poem was a great deal.