Out of the fire and the dense clouds, the LORD spoke those words to your whole congregation at the mountain, with a mighty voice that did not continue/end. He inscribed them on two tablets of stone, which He gave to me.

ולא יסף — We render this in the Targum by ולא פסק “and He did not stop” for His voice is strong and goes on continuously. Another explanation He did not again ever reveal himself with such publicity.

Similarly it says, and they prophesied ‘v’lo yasafu’. And in line with the plain meaning of Scripture v’lo yasaf means that “there will never again be so great [a voice].” But by way of Kabalah the verse is stating that they heard all these words [proclaimed by] the voice of the Great One, and He did not add anything [perceivable] to all your assembly, except for this one voice, for it was only that voice that they grasped.

ולא יסף, according to the plain meaning of the text there was never again such an overpowering sound in the history of mankind. Whenever the word יסף appears it represents an addition of something.

The idea is as is written, "If you listen, hear", as Rashi interprets: "Listen" to the old, "[you will] listen" anew. The idea is as is written, "A mighty voice that did not end" - it did not cease, for the words are living and persisting. The Holy Blessed One gives the Torah always. By keeping the Torah and commandments, you can hear new things in each moment.