Why we cant able to speak on moon or vistle on the moon or in space ???
Sound propagates as longitudinal waves and it requires a medium such as solid, liquid or gas to travel.
Since space is devoid of a medium (it's vaccum) sound cannot travel in space.
The same concept applies for moon also. There is no atmosphere in moon and hence there is no medium for sound to
Thus, it is not that we can't speak in space or moon. We can speak!! but the vibrations produced/sound doesn't reach others.