What the speaker hears is the call of Innisfree and the simple life lived close to nature that it represents. The speaker is profoundly dissatisfied with urban life, which is somehow less real, less authentic than the isolated existence out in the wilderness that he craves. Even in the midst of all the hustle and bustle of the city, the speaker can still hear the call of the wild, feeling it deep within the very depths of his soul. His previous experience of life on the lake isle has clearly had a profound effect upon him, so much so that he has been changed forever by his brief taste of the simple life and its equally simple pleasures.
Though forced to return to the city, the speaker knows that he cannot ignore the insistent call of Innisfree for very long. The remote, rugged landscape has entered his soul, and there it shall remain forever, insistently calling upon the speaker at regular intervals to renew his almost mystical connection with the soil of this most blessed plot of earth.