in "Blues Ain't No Mockingbird," the two hawks do symbolize Granny and Granddaddy Cain because they are attacked in their own environment. Granny and Granddaddy want the film crew to leave, stating simply that "This is our own place." The film crew looks at the Cains with indifference, thinking they are a stereotypical poor, suburban/rural family on food stamps. This stereotypical prejudice is also based on a racist assumption that the Cains must be on welfare, despite the observation of Smilin man that Granny raises her own vegetables.
Just as Grandaddy kills the hawks, the film crew intends to "shoot" the Cains for their documentary. It is ironic then that the Cains symbolize the hawks since it is Granddaddy who kills them.
However, the hawks might also symbolize the film crew and the camera itself. The camera is described as "buzzin" which symbolizes the hawk's screech. When the hawk comes looking for its mate, it must be killed. Shortly thereafter, Granddaddy smashes the camera. The Cains see the hawk and the camera as threats to the privacy and comfort of their home. So, it's not a clear case where the hawks symbolize only the Cains. But the comparison between the hawks and the Cains does seem to be the primary analogy.
The reference to Goldilocks symbolizes the film crew. The Cains are the bears and Goldilocks is the film crew of white men (gold locks being more characteristic of whites). Goldilocks intrudes on their land. Cathy doesn't finish the story, so the reader is left to wonder whether the intruders will be thrown out. Eventually, Granddaddy does just that.
The hawk's screech is symbolic of the lament of the blues. The blues the Cains figuratively sing are about the trouble they face in dealing with ignorant people like the film crew. The crew speaks condescendingly (mockingly) toward Granny and Granddaddy, but they don't let it affect them. Their words (the blues or their call - hawks) will not be mocked. They will proudly defend their place.