I just read a cool article that experimented with voices coming from a ceiling here. Here's another experiment I set up recently, in my house, to experiment with voices heard on the other side of a wall. It's like a magic trick.
1) set up a laptop A against a physical wall.
2) hook up laptop A’s sound output to a bluetooth speaker on the other side of the wall.*
3) set up a google hangout between laptop A and another laptop located in another room B against its wall.
With this setup A looks like a window on the wall, looking into B. It also sounds like the conversations in room B are being overheard by you as if the people in room B are speaking to each other on the other side of the physical wall. The result is the feeling that room B and room A are on opposite sides of a real wall, but the sound tricks you into thinking people are conversing just on the other side of a wall, with the sound spilling into your room from the hallway outside.
n.b. #1: This is more useful for voyeurs than it is for people looking to communicate between the two rooms.
n.b. #2: I came up with this experiment after watching "The Lives of Others" and then setting up this quick experiment to "peer" from my kitchen into my living room, during the NFL playoff game last night.
n.b. #3: I read here that this may be just another configuration of "telepresence."
* or, locate the bluetooth speaker on the other side of the door to the hallway, although this isn't as effective