I've read transcripts of what the voices are saying and it seems to be they are monitoring what's going on, but not in control. It think I read somewhere that when Walt is talking to Shannon (sssshhhhh! and dripping wet) he is actually speaking backwards. What's the deal there?

In season two when Eko was tapping on trees searching for the ones "he liked" for the church I wondered if , because of the tapping, he was seeing if they were hollow or something, like maybe they contained microphones, speakers, or small cameras to spy on that island. That's why people in the jungle could hear the voices. Just like the speaker in Jack's aquarium that "hadn't worked for 20 years," yet we heard voices on it. Maybe the tree speakers didn't work perfectly either and the sound could be heard as whispers. It had just been my thought at the time.