Several things here
(aside from the fact that this is an old thread)
1: You aren't God, so where do you get off supposing how He would make things?
2: Man was given domain over all things on this planet. It stands to reason that this means that all animals recognize us as their immediate masters. And what servant doesn't know the voice of his master?
3: Just because you haven't heard a response about your suppositions, doesn't mean noone is talking. It may mean you aren't asking the right people.
4: According to evolution, such a "useless" mechanism in rats (been around for eons before language) should be gone by now.
EXCELLENT POINTS! I especially loved #4. Evolution as preached (more like dogma than any Creationist view) should have eliminated the rat's ability to discern human speech millions of years ago. Why is it still around?