The word "voice" here refers to a sound that God made. Possible meanings are 1) "by speaking loudly like thunder" or 2) "by speaking" or 3) "by causing thunder to sound" (See: [[rc://en/ta/man/translate/figs-metonymy]])
God spoke about walking past the boundary as if they might break down a barrier and walk through it. See how you translated "set bounds" in [Exodus 19:12](./12.md). AT: "not to go beyond the boundary" or "not to go through the barrier"(See: [[rc://en/ta/man/translate/figs-metaphor]])