We know that the sound waves propagate through air, and it can't travel through vacuum. so the thing that help it doing that is the air's molecules pressure. So my question how can that happens? I can't understand that concept.
Sound waves propagate very similarly to how 'the wave' propagates at baseball stadiums: http://www.youtube.com/watch?v=H0K2dvB-7WY
At some point something (your vocal cords, a piano string, a speaker) hit a bunch of air particles (atoms, molecules, it really doesn't matter). These particles in turn hit the particles next to them, these hit the ones next to them and so on. No pressure here is simply the absence of any particles, so nothing communicates the orders to move. This is like ' the wave' in that everyone communicates the motion of the wave of the person standing next to them, and if there is no one standing next to you, the wave ends with you. Hearing a sound is the last bunch of air particles next to your ear drum getting the instructions to vibrate which in turn vibrates your ear drum and your brain turns this response into the perception of sound.
Sound waves are mechanical waves. They can only travel through solids, liquids and gases. In all of these three forms of matter, sound waves are able to transfer energy (sound) through vibrations of its particles such as molecules. For the case of gases, compressions and rarefactions help transfer sound.