Sound needs medium to travel from the source of sound to our ear because the sound is created by vibrations and vibrations need a medium to travel. Whether it is solid, liquid or gas, sound travel in all medium. Without medium sound cannot travel. Sound travels in the form of compressions and rarefactions in the medium. We can also test it experimentally. We all move in a car, notice that when the car is totally packed, we cannot hear any sound from outside.
We all know that in space there is no medium in which sound can travel. There is no solid, no liquid and no gas. So, on the moon, we cannot hear any sound. Therefore, astronauts communicate with each other by signals or microphones in space even they are closer.
