In "Dover Beach" what is it that Sophocles might have heard, literally and figuratively, on the Aegean Sea, according to the second stanza?

These are the lines from Arnold's elegiac poem, Dover Beach you refer to in your question :

Sophocles long ago

Heard it on the Aegaen, and it brought

Into his mind the turbid ebb and flow

Of human misery, we

Find also in the sound a thought

Hearing it by the distant northern sea.

Arnold, looking at the nightscape of the Dover sea--the 'moon-blanched' sandy shore, 'the long line of spray', the image of the moon on the straits, the glimmering light on the French coast, the upcoming & retreating waves--and listening to harsh frictional noise of pebbles in the tides, remembers the ancient Greek tragedywright, Sophocles, who must have also heard the same 'eternal note of sadness' in the tides of the Greek sea, as Arnold hears in the Dover sea in the middle of the 19th century.

The lines show a literal as well as a symbolic switch to the past from the present. Arnold, an avid reader and admirer of Classical Greek literature, writing an elegy on the tragic decline of faith in the world of man in an age of Industrialism, finds proximity with Sophocles. At the heart of Sophocles's tragedies was a deep perception of 'the turbid ebb and flow/Of human misery' which his successor can still hear in a different time and space. The auditory image of the 'grating roar' in the perpetual movement of the sea-waves brings into the mind of the English poet of the Victorian era an 'eternal note of sadness', and it was not unknown to the ancient writer of tragedies. Thus Arnold believes that he himself and his illustrious predecessor were both artists working out universal tragic predicament.