Sound is the wave of air molecules caused by the motion of an object. No movement means no sound. If the object moves faster and vibrates more, it means higher frequency and louder the sound.
Sound travels at different rates in different media. In dry air, sound travels at 331.6 meter/sec at 0 C.
The most common sound unit is decibel (dB), it's 1/10 of bel (B). The other sound unit "neper (Np)" is rarely used.