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.
What is the meaning of "ve'Lo Yasaf"?
Rashi #1 (citing Targum Onkelos): It means that the Voice (did not get louder, because it) did not cease. 1
Rashi #2: It means that Hashem never appeared again with the same pomp and ceremony as He did on Har Sinai.
Ramban #1 (according to Targum Onkelos and Targum Yonasan): It means that Hashem's Voice did not stop (or recede) - that it retained the same volume 2 from the beginning of the Aseres ha'Dibros until the end. 3
Ramban #2 (according to Kabalah): They heard all these things (with reference to the last eight Dibros) with a Great Voice (which they could not comprehend) 4 , and Hashem did not continue to let the congregation hear with the One Voice (the first two Dibros, which they did comprehend).
Rashbam: There was never again a great voice such as this.
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 Kol got louder and louder" (refer to Shemos 9:19:1:1), it is with reference to the tone of the Shofar, and not to the Voice of Hashem.
QUESTIONS ON RASHI
Rashi writes that Hashem never appeared again with the same ceremony. In Sanhedrin 17a, it says that "v'Lo Yasafu" (Bamidbar 11:25) means that they ceased to prophecy, and challenges this from our verse!
Riva citing R. Elyakim: It challenged 'what is the source that [except for Eldad and Meidad, the others] totally ceased?' Here, the voice continued, just not with the same ceremony! It answers that since v'Lo Yasafu is written only about the others, it means that they totally ceased.