Let me find out.
Have NOT found the answer yet.
This is probably the infamous first question which we can't find an answer for.
We have searched online for a clue or an explanation but nothing.
We know that in the first or second episodes the actually say a few words and in later episodes they mumble sounds, but no speak for the most cases.
The intriguing part is that other characters speak, either humans or animals.
what was the decision behind keeping the main characters mute?
In any case its an interesting question that had never come up in my mind until Ander asked last weekend.
We knew there would be one question one day that would challenge this site. This is that question. Any light that would help find an answer would be appreciate it.