1.Find a room with a window you'd like to spy on, and aim the laser at the window at an angle.
2.Determine where the laser reflects, and situate your photocell so the reflected beam hits the front of the cell.
3.Hit record and listen!
4.Any noise from inside the room will cause the window to vibrate, which will get picked up by the laser and photocell. You might have to process the raw input to remove unwanted noise and isolate the voices.