What exactly does "mi'Toch ha'Eish, he'Anan ve'ha'Choshech" mean?
Ramban: It means from within the fire which was surrounded by darkness and the thick cloud.
And what is the meaning of "ve'Lo Yasaf"?
Rashi #1 (citing Targum Onkelos and Targum Yonasan and Ramban #1: It means that the Voice (did not add, because it) did not stop. 1
Rashi #2 and Ramban #2: It means that Hashem never appeared again with the same pomp and ceremony as He did on Har Sinai.
Ramban #3 (according to Targum Onkelos and Targum Yonasan): It means that Hashem's Voice did not stop (or recede) - that it retained the same volume from the beginning of the Aseres ha'Dibros until the end. 2
Ramban #4 (according to Kabalah): They heard all these things (with reference to the last eight Dibros) with a Great Voice (which they could not comprehend) 3 , and Hashem did not continue to let the congregation hear with the One Voice (the first two Dibros, which they did comprehend).
Rashbam: Hashem never again added to that unique revelation, 4 (to reveal Himself again as He did there).
Rashi: Because, unlike a human-being, who cannot say all that he wants to say in one breath, and who therefore, needs to breath again before continuing, Hashem is able to say everything in one breath; consequently, He does not need to breath a second time in order to continue and His Voice retains its initial volume. Ramban: And when the Torah writes in Yisro that "the Voice got louder and louder" (refer to Sh'mos 9:19:1:1), it is with reference to the tone of the Shofar and not to the Voice of Hashem.
Rashbam: As we find later, in Pasuk 22.