What has the Sea of Faith done in the poet's age? What is the only sound that he hears? What was the sound the sand made?

The Sea of Faith is in retreat. Arnold hears its "melancholy, long, withdrawing roar" as the ebbing tide recedes over the shingle beach. The world of Victorian religious certainty is going out with the tide, yet still it lets out a proud, final roar, at once defiant and sad. Although it is still possible to hear the "grating roar" of the pebbles as the waves pull back, there is no doubt that high tide—the assertive, confident Christian faith of mid-Victorian Britain—has begun an irreversible retreat.
In the midst of all this epoch-ending melancholy and confusion, there is still a place for love. And to their love, Arnold and his new bride must be true, building a life for themselves among the darkened ruins of a slowly dying age.