Answer:
When two astronauts are inside the same vehicle, capsule, shuttle, space station, or anything else
where they don't need helmets and oxygen masks in order to breathe, they communicate by speaking,
just as they do when they're in an office on earth.
When they're in a place with no air, such as during a space "walk" or on the surface of the moon,
they communicate by radio, even if they're standing next to each other.