What the speaker "hears" in his deep heart's core is the sound of the lake water lapping against the shores of the lake isle of Innisfree. This line shows—or suggests—that the isle home he thinks about in the middle of the lake is imaginary. We know he is imagining the sound of the water because he says he hears it
While I stand on the roadway, or on the pavements grey
In other words, he hears it while he is the midst of the city and a trafficked roadway. By saying he hears it in his "heart," he means he doesn't literally hear it; he can't really hear the sound because he is too far away from that peaceful setting. It is something he remembers or imagines that gives him comfort and peace amid the busy pace of life.