:o why do we get a echo sound in empty rooms? :p

In order to get an echo, sound must follow a rule of reflection that says that it will bounce at the same angle at which it hit. If you expect any sound that hits the wall directly to bounce straight back to your ear, it must follow this rule. But it will only work if the bouncing surface is smooth. That's the second requirement for a good echo.

Echo Echo Echo Echo Echo Echo Echo Echo Echo Echo Echo Echo Echo..........

Echo (phenomenon - Wikipedia, the free encyclopedia)

An echo can be explained as a wave that has been reflected by a discontinuity in the propagation medium, and returns with sufficient magnitude and delay to be perceived.

When dealing with audible frequencies, the human ear cannot distinguish an echo from the original sound if the delay is less than 1/10 of a second. Thus, since the velocity of sound is approximately 343 m/s at a normal room temperature of about 20°C, the reflecting object must be more than 16.2 m from the sound source at this temperature for an echo to be heard by a person at the source.

Propagation Medium
Any compressible or deformable substance, which pretty much describes all things on earth. The molecular makeup of a substance defines its capability to propagate sound. While air and water are the most commonly mentioned sound propagation media, they are simply the most abundant substances on our earth's surface. The density and compressibility of air and water are such that they change the characteristic of sound very little as sound waves propagate through them.

Steel is a relatively incompressible medium; a decimeter thick wall is pretty much a barrier to sound propagation. However, a millimeter thick membrane is deformable and thus able to transfer compression in the medium on one side to the medium on the other. The deformation of the membrane is transient and thus propagates across the membrane causing residual compressions in the media on both sides of the membrane. The result is that as a propagation medium steel changes the characteristics of sound greatly.

From: AuSIM: Glossary P-R

It's a bit difficult to explain. Essentially, sound is bouncing off the walls of the room, either because of it's size or the composition of the walls, that allow sound to return to you as an echo.