A sound wave is a transfer of energy as it travels away from a vibrating source. Sound waves are formed when a vibrating object causes the surrounding medium to vibrate. A medium is a material (solid, liquid or gas) which a wave travels through. As sound waves move through a medium the particles vibrate forwards and backwards. A sound's volume, how loud or soft it is, depends on the sound wave. The more energy put into making a sound or a sound wave, the louder it will be. The farther a sound wave travels, the more it spreads, this makes it more difficult for us to hear a distant sound. So the nearer we are to a sound the louder it sounds to us. A sound wave enters the ear and is changed into nerve signals, which are interpreted by the brain.
As a given object vibrates or oscillates in air, for example, the air molecules near or around the medium will be moved back and forth in relation to the frequency and force of the vibration.
This energy then moves through the air as a pressure wave.
If the vibration is strong and slow, like a bass drum hit with a soft mallet, the sound will be loud and low in pitch, if the vibration is weak but fast, like a pin falling on a wooden floor, the sound will be soft and high pitched.
The air molecules themselves don’t move across the room, much like a water molecules on one side of a swimming pool do not move to the other side in a wave, it’s the energy that moves through the medium.
This is why there is no sound in space, there is no medium for the sound wave to move through.
Think about it, you cannot make water waves in an empty swimming pool.
Here’s where it gets interesting. Once these vibrations are propagating through the air, can it be called sound?
After all it is just energy moving through a given medium, so when does sound become sound?
From one perspective the only thing that makes sound exist are ears, human or otherwise, and nervous systems and brains (or ganglia) to interpret and decode those vibrations into meaningful data - i.e., sound.