The African American Vernacular English in which Deacon addresses the southern Quentin contrasts sharply with the standard American pronunciation he uses when speaking to Northerners. Though the precise roots of this dialect remain obscure, it has many similarities to Southern American English — unsurprising, given the strong historic ties between slavery and the South. In the pre-Civil Rights era, attempts to render it in literature are largely inextricable from the racist stereotype of African Americans as lowly and intellectually simple. Faulkner's intentions, however, are much more subtle and have to do with the way in which race was constructed and enacted. This is evinced not only in Deacon's opportunistic flipping between registers but also through the differences in how white characters hear black speech. It is notable, for instance, that Benjy perceives little difference between black and white language whilst in all other sections they are quite distinct.
This 1937 take on Uncle Tom's Cabin by Tex Avery presents a whole array of stereotyped black speech. It was banned in 1968 for its racist caricatures.
A more authentic recording, a 1941 interview with George Johnson, a former slave from Mississippi, can be heard here.