The speaker's tone in "Harlem" is best described as frustrated.
The poem's imagery helps to convey this tone. In discussing a deferred dream, Hughes describes a dried up raisin in the sun; a festering sore; stinking, rotting meat; and a sagging, heavy load. At the end of the poem, he wonders if the deferred dream just explodes.
This imagery helps provide the key to understanding the speaker's attitude, or tone, about his subject, the deferred dream. He is frustrated that these dreams are wasted.