And mouth with myriad subtleties. (5)
There's a whole host of "subtleties" that play into the different categories of society and class, especially when you're dealing with the highly volatile world of racial prejudices in Dunbar's poem.
Nay, let them only see us, while We wear the mask. (8-9)
The world in Dunbar's poem only sees the class distinctions that society has created based on race and those associative "masks."
We sing, but oh the clay is vile Beneath our feet, and long the mile; (12-13)
The "clay" here indicates the literal earth, but it also hints at questions of one's origins. So if it's "vile," we can assume that the society that it's part of isn't doing any justice for the people who have to suffer the inequality of that particular culture.