Chapter 22 is a big one for symbolism.  The very idea of a confrontation between Assef and Amir is symbolic of the battle between good and evil.  Amir has spent the entire narrative seeking to "become good again," while Assef has been fairly consistent in his own sense of pure malevolence and evil being on display for all to see.  The fight between both of them is reminiscent of the sodomy of Hassan by Assef.  In that fight, Amir was on the periphery watching and not taking action.  In this confrontation, Sohrab is on the periphery, but represents a reconfigured symbol of Amir as a child, for he takes action with the slingshot against Assef.  The symbol of someone on the margins of discourse using their voice is represented by Sohrab in this chapter.  At the same time, I think that it is evident that the abuse that Amir suffers is an attempt to make him more like Hassan during his own moment of abuse.  The hairlip that emerges is symbolic of this and in the process, the silencing of voice is something that Amir represents, similar to Hassan experienced earlier in the narrative.