Well technically we have "voices" already, but only for special NPC dialogue, and they're not actually saying anything we can understand, just each one has a unique style of audio bleeping that represents them. I wouldn't want any English in there though, for sure. Not the right atmosphere for that. Now maybe one day if there are humans involved... ;)
Cue wild speculation involving extended game spoilers:
1. Fairly safe: MC is wrong and humans are not extinct.
2. Human military forces are closer to technology parity with the aliens than the robots in the game. After all, humanity managed to at least 10 years in the solar system if I recall the relevant lore correctly. It would make sense that they'd create MC as a trap and a sitting duck to make it look like humanity is an easier opponent than they are. Also, you don't really want your own weapons turning against you so you probably wouldn't give them the highest level of firepower. The most powerful weapons we see in the game are ST and the singularity machine. (Even the BFG, for all its single target damage, isn't that crazy.)
3. Just crazy: Robo Zhirov has edited memories and isn't who he believes himself to be. Perhaps human Zhirov, or companions, are still alive. Would you really leave a military installation in the charge of a single elderly scientist? The robots could all have deeply implanted human-masking routines in their firmware, so that humans can observe their operations without interfering.