This is fairly interesting to examine. I think that the speaker in the poem could be Hughes, himself. The idea of Hughes positing himself in his poem could be quite plausible. Given the background of articulating in his own mind the condition of African Americans in the early 20th Century, he certainly would be able to speak quite lucidly to the implications of deferred dreams. Additionally, given the fact that Hughes was one of the first thinkers to give voice to the experience of African Americans as one that had a large impact on regionalism (Southern African Americans and African Americans in the North), such a perspective would effectively allow him to comment on the deferral of dreams in different settings. Another potential explanation of the speaker could very well be someone whose dreams have been deferred repeatedly over time. This can speak loudly to the idea of the intricacies in the deferral of one's dreams and how one perceives such elements in a myriad of ways, such as agony, forlornness, quiet despondency, or intense displays of anger.
I would encourage you not to see the poet Hughes as the speaker in the poem "Harlem." It's certainly fair to see overlap between the two, but the "I" in the poem -- much like the "I" in "The Negro Speaks of Rivers" and in many of Hughes' other poems -- may very well be much larger than a single individual.
In Burns’s poem, a gentleman is speaking to a female (his lass). The first stanza is a brief general statement to whoever may be near, including the speaker’s lass herself. In stanza two, and in the remainder of the poem, the speaker addresses the lass directly, explaining to her that even though he must travel, she will always be foremost in his thoughts because he attributes to her the beauty of both flowers and music. The speaker asserts that seas must dry and rocks must melt before his love will end. These are both figures of overstatement, and they suggest a strong commitment. More serious as a figure is the metaphor of the “sands o’ life,” which compares life to an hourglass that will eventually run out of sand. Though the concluding metaphor of lengthy travel (we might remember that at the time a trip of ten thousand miles might have taken three or four years) is also hyperbolic, the sands metaphor suggests that the speaker, underneath his exaggerations, is not without his serious side.