The best way to hear through a wall is to drill a pinhole through it and then place plastic wrap over the hole. The plastic almost completely eliminates the sound barrier created by the wall. This method requires some expertise.
The Japanese and Korean researchers who tested the pinhole method caution that it is not for amateurs, however. Sizing, spacing and tuning all affect the quality of the sound. The results of the pinhole experiment are most likely to have the most significant impact on security methods for facilitating sound through walls. A thin glass turned upside down also amplifies sound through walls. The science behind the glass to the wall is pretty simple. Sound travels in waves. Sound waves never completely dissipate. Rather, they're converted into other energy. Because a wall has a higher density than air, the wall absorbs the sound coming from within the room. This causes the wall to vibrate. At normal decibel levels, the vibrations is so minute as to be unnoticeable. When the noise is particularly loud, however, it is possible to hear the walls vibrating. Thus, there is validity to saying that something was so loud "it shook the walls." When a thin glass is held against a wall, however, it places just enough pressure against the wall as to stop the vibrations in that spot. Thus, it is easier to hear through the wall.