When Moses would come into the Tent of Meeting so G-d could speak with him, he would hear his voice.
Much as we might wish it, we cannot be allowed to hear G-d's voice everywhere and at all times. If we could, we would be deprived of our freedom of choice. A world in which G-d's voice is constantly heard does not challenge its population.
It was G-d's desire to create a world of Divine silence, in which, through our efforts, we can uncover G-d's concealed voice.
It is our task to take what we heard during that short period at Mount Sinai and within that small space of the Tabernacle--and each of us has heard G-d's voice somewhere and at some time, however fleetingly--and transmit it to the rest of time and space.