Nahmanides’ commentary to Exod. 20:2

Jewish Mysticism
[The Torah] was given to Moses in seven voices [i.e., the seven sefirot], which he heard and comprehended. But with respect to Israel, they heard it in one voice [i.e., the Shekhinah], as it says, “a loud voice and no more” (Deut. 5-19). And it says, “You heard the sound of words but perceived no shape–nothing but a [single] voice” (ibid. 4-12). And here too [Scripture] alludes [to this]- “And all the people saw the voices” (Exod. 20-15), the word qolot (voices) is [written] without a waw [signifying the plural form], for they [Israel] saw all the voices as one [i.e., the Shekhinah].

Translated by Elliot Wolfson in Through a Speculum that Shines- Vision and Imagination in Medieval Jewish Mysticism, Princeton- Princeton University Press, 1994.