In the second strophe the poet is assured that if "they," the community, complain of the poet's practicing "arpeggios," or breaking the past's unities into the present's pieces as arpeggios break unified chorads into sequences of individual notes, it is because they are too involved with the dead past--the "body in rags" they are carrying "down the stairs" (to burial, presumably)--to want the reality of things as they are now. But the poet is to play despite them.
At this point the speaker breaks off into a revery about the past which he characterizes as "lucid" and playful: a "divertimento," or suite of dances, adn about the imagined future, also lucid, or "unclouded," and structurally solid and impressive--a concerto rather than the "hoo-hoo-hoo" the poet is to play. But it is snowing--winter bleakness is here; hence, the poet is to forget past and future, divertimentos and concertos, and play the present. He is to lose his identity voicing that present's "angry fear" and "besieging pain." And he is to be a "Thou," or bardic/prophetic and highly personal voice rather than a small impersonal "you." In doing this, he is to become so universally & forcefully representative of sorrow that those hearing him will forget themselves and thier sorrow--they will experience, I would guess, a catharsis and the poet will, figuratively, blow their sorrow into the stars, or beyond earthly stress into peace. Once this has been accomplished, we might be able to return to the clarity & classical "rightness" and youthfulness unencumbered that Mozart's music represents. Meanwhile, however, we are old & suffering. Therefore, the poet must "be seated"--and play. Art will cure us of sorrow by expressing the objective now.