I listened to it in English, and I thought the voice acting was fine. I wonder, the character without the mouth, is there a way to show he is the one talking? This is often done several ways with a character that doesn't have a mouth: moving his head, moving his eyes, using some sort of flashing lights, etc.
As for the background talk, it wasn't annoying as much as distracting. Is the audience supposed to here specifically what is being said? In my opinion, you should keep the volume a little lower until a few moments before the middle character complains about it to the one watching it.