Here "voice" is a metonym for the power that the slave drivers have over the slaves. Alternate translation: "They are no longer under the control of the slave driver" (See: [[rc://en/ta/man/jit/figs-metonymy]])
This refers to slave drivers in general, not one particular slave driver. Alternate translation: "slave drivers" (See: [[rc://en/ta/man/jit/figs-genericnoun]])