The Torah tells us that when Moses went into the Tent of Meeting to speak with G-d, he would hear a voice addressing him from above the cover that was on top of the Ark.
G-d's voice could not be heard outside the Tent. The same tremendous voice that spoke at Sinai stopped short at the door of the Tent and did not travel further. At Sinai and in the Tent, G-d's voice was stopped. At Sinai, after the revelation, a shofar, horn, was sounded signaling the departure of G-d's presence and voice -- a time-based limitation. In the Tent, this cessation was space-related, the voice reaching a certain point and not going any further.
Much as we might want it to be otherwise, G-d's voice cannot resound everywhere and at all times. If it did, we would not have freedom of choice. A world where G-d's voice is constantly heard does not challenge us. It was G-d's desire to create a world where we uncover G-d's concealed voice through our own efforts. Our task is to take what we heard at Sinai and in the Tent - each of us has heard G-d's voice in some place and at some moment, however fleeting - and carry it over to all times and all places, wherever we may be.