Question

Why it is difficult to hear each other voice clearly in empty and unfurnished room

Answer 1

Because in an empty room the  sound produces multiple reflection since the sound absorbing materials (curtains, furniture, etc) aren't present

Answer 2

The best acoustics is made with the help of a great balance between the absorbent and the reflective surfaces within the same room.

The furniture in a room usually absorbs the waves of the sound, and no echo can be heard.

However, in case of an empty room, the voices echo without being absorbed which makes it difficult for the hearer to hear.