The Question and Answer section for Matthew Arnold: Poems is a great resource to ask questions, find answers, and discuss the novel.
I think this would be the sound of the pebbles in the waves which brings the speaker to think of Sophocles and ancient Greece.
Please check out Grtadesaver's summary analysis for a detailed look at the comparison to Sophocles' experience on the Aegan Sea. The necessary link can be found at the top of the page.
The speaker warns that the scholar-gipsy should avoid everyone who suffers from the "disease," of man lest he become infected as well. Arnold feels weighed down by the responsibilities of life. He wishes, like the scholar Gipsy, he could escape...