The sound of the tide, the "grating roar of pebbles which the waves draw back, and fling" back up onto the beach as the waves rush up the shoreline and then retreat, creates many interrelated emotions in the narrator.
The most general overall description of the emotion being felt refers to "the eternal note of sadness." In the sound of the tides endlessly washing back and forth, the speaker imagines he hears the age-old patterns "of human misery." The rocks are unceasingly thrown onto the beach and then washed back into the sea; there is no respite from the battle taking place between the rocky ground and the power of the moving water.
In this ongoing conflict, the speaker hears the loss of tranquility that the "Sea of Faith" had once transmitted to the world. All that is left, in the narrator's impression of the sound of the waves, is the "melancholy, long, withdrawing roar" of the world as the "land of dreams" that once existed is washed away and destroyed by the inescapable power of the wind and water.
In the opening lines of the poem, the speaker associates the sound of the tide with a feeling of tranquility. This is because the sea is "calm" at this moment, creating a sense of inner peace. But, as this stanza progresses, the emotions begin to change. Suddenly, the sound of the tide is described as a "grating roar" as it hits the pebbles. The word "grating" suggests that it is an unwelcome and unpleasant sound that destroys the initial sense of peace in the speaker's mind. Moreover, as the tide continues its ebb and flow, the speaker notes that a feeling of "sadness" begins to creep in.
This sadness is further developed in the third stanza. Specifically, this is depicted through the use of the world "melancholy" to describe the roar of the tide. In addition, this roar is described as being "long" and "withdrawing," which suggests that this is a prolonged bout of sadness; it shows no sign of relenting.
So, for the speaker, the changing sounds of the tide reflect his descent from calm to a profound sense of sadness.