Listen! you hear the grating roar (10)
This is the first inkling we get that things might not be all good in the speaker's world. Up until this point, we've been reading what feels like a pretty chilled-out nature poem. And then suddenly, the word "grating" pops up. That's a kind of unpleasant word, both in the associations we have with it and just in the way it sounds. It's like a guitar string breaking in the middle of a song. We can't ignore it, and suddenly dark thoughts are creeping into the poem.
With tremulous cadence slow, and bringThe eternal note of sadness in. (13-14)
Here the speaker actually comes out and connects the sound of the ocean to "sadness." Most important, that sadness doesn't just pop up here and there. It's eternal, something human beings can never get away from.
Its melancholy, long, withdrawing roar, (25)
In this poem, sadness is all about sound. The world looks calm and beautiful, but it sounds just miserable. This line is a great example of that. The metaphorical "Sea of Faith" (21) looks great when it's at high tide. But when it starts to desert the world, it makes a terrible, sad, "melancholy" sound. We don't see the sad event taking place—we hear it.