Sophocles long ago Heard it on the Ægean, and it brought
Now the sound of the pebbles in the waves turns into a kind of time machine, and takes the speaker (and us) on a mental journey back to ancient Greece.
He imagines the famous playwright Sophocles heard the same sound as he stood next to the Aegean Sea (that's the part of the Mediterranean that separates Greece from Turkey).
This little allusion to the past keys us into Arnold's interest in the past, and especially classical Greece and Rome. It also creates a connection between the great poetic mind of Sophocles and our speaker. They are linked, across the centuries, by the act of listening to the sea and thinking about humanity.
Into his mind the turbid ebb and flow Of human misery; we
Sophocles was one of the great Greek authors of tragic plays. You know, those bummer dramas where everyone ends up dead or miserable. So, it's probably not that surprising that the ocean makes him think of "the turbid ebb and flow of human misery." "Turbid" means "cloudy, stirred up, muddy and murky" and it's often used to refer to water.
So, Sophocles is imagining an analogy between human unhappiness and cloudy water moving in and out ("the ebb and flow").
Also, have you been keeping an eye on how much enjambment this poem has? This particular stanza (lines 15-20) is just one long sentence broken up over six lines. This makes the connection between the distant past and the present seem almost seamless.
See how he slips that "we" in at the end of line 18? He's zooming us back to the present, without even ending the sentence. He could have easily stopped and started the next line back in the present (although breaking it up the way he does helps with the iambic meter). Instead, he just zips back, without stopping, forcing us to keep moving at his pace.
Find also in the sound a thought, Hearing it by this distant northern sea.
Now we're fully back in the poem's present, back on the shore of the English Channel. Here he calls the Channel "this distant northern sea." By distant he just means far away from Sophocles and the Aegean.
Just like Sophocles, "we" find a thought in the sound of the waves. Who's this "we," by the way? Line 18 is the first time the speaker has referred to we. Maybe he just means him and his companion (whom he invited to the window in line 6).
We've got a hunch he means something bigger though. If it was just he and his companion, he wouldn't need to talk about it.
We think he's including a lot of people in his "we"—his readers, and maybe all of the people living in his time and place. It's a way of both drawing us in and making his observations seem universal.