Tone and mood are two different things. Tone is the attitude of the narrator/speaker, and mood is the emotion or feeling that a piece gives to readers. While those two things are different, they do share a close relationship because an author's tone often leads the mood that readers experience. This is true of this poem, but the tone and mood are not the same throughout the entire poem.
The first stanza starts off quite calm and peaceful. Shelley uses words like "warm," "clear," "bright," and "dancing." It evokes happiness. We are even told that the city's voice is "soft like Solitude's." It is an uplifting stanza. However, this does not continue. The remaining stanzas pull the reader's initial happy feelings down toward a more dejected and depressed mood. This is the poem's goal. The speaker is describing his feelings, and the reader is along for the ride. What is especially interesting is how the day's weather begins to turn with the poet's tone and the poem's mood.
The lightning of the noontide oceanIs flashing round me, and a toneArises from its measured motion,How sweet! did any heart now share in my emotion.
Alas! I have nor hope nor health,Nor peace within nor calm around,Nor that content surpassing wealthThe sage in meditation found,
They might lament—for I am oneWhom men love not,