How can a G-d without form produce sound (voice)? For instance, G-d spoke at Mount Sinai to millions of Jews.
As explained by Rambam, God does not actually have a "voice". God is simply described that way because that is how the human mind can understand the nature of communication.
Guide for the Perplexed 1:46
The perception by the senses, especially by hearing and seeing, is best known to us; we have no idea or notion of any other mode of communication between the soul of one person and that of another than by means of speaking, i.e., by the sound produced by lips, tongue, and the other organs of speech. When, therefore, we are to be informed that God has a knowledge of things, and that communication is made by Him to the Prophets who convey it to us, they represent Him to us as seeing and hearing, i.e., as perceiving and knowing those things which can be seen and heard. They represent Him to us as speaking, i.e., that communications from Him reach the Prophets; that is to be understood by the term "prophecy," as will be fully explained. God is described as working, because we do not know any other mode of producing a thing except by direct touch. He is said to have a soul in the sense that He is living, because all living beings are generally supposed to have a soul; although the term soul is, as has been shown, a homonym.
Again, since we perform all these actions only by means of corporeal organs, we figuratively ascribe to God the organs of locomotion, as feet, and their soles; organs of hearing, seeing, and smelling, as ear, eye, and nose; organs and substance of speech, as mouth, tongue, and sound; organs for the performance of work, as hand, its fingers, its palm, and the arm. In short, these organs of the body are figuratively ascribed to God, who is above all imperfection, to express that He performs certain acts; and these acts are figuratively ascribed to Him to express that He possesses certain perfections different from those acts themselves. E.g., we say that He has eyes, ears, hands, a mouth, a tongue, to express that He sees, hears, acts, and speaks: but seeing and hearing are attributed to Him to indicate simply that He perceives. You thus find in Hebrew instances in which the perception of the one sense is named instead of the other; thus, "See the word of the Lord" (Jer. ii, 31), in the same meaning as "Hear the word of the Lord," for the sense of the phrase is, "Perceive what He says"; similarly the phrase, "See the smell of my son" (Gen. xxvii. 27) has the same meaning as "Smell the smell of my son," for it relates to the perception of the smell. In the same way are used the words, "And all the people saw the thunders and the lightnings" (Exod. xx. 15), although the passage also contains the description of a prophetical vision, as is well known and understood among our people. Action and speech are likewise figuratively applied to God, to express that a certain influence has emanated from Him, as will be explained (chap. lxv and chap. lxvi.)(Friedlander translation)
Guide for the Perplexed 1:65
AFTER YOU have advanced thus far, and truly comprehended that God exists without having the attribute of existence, and that He is One, without having the attribute of unity, I do not think that I need explain to you the inadmissibility of the attribute of speech in reference to God, especially since our people generally believe that the Law, i.e., the word ascribed to Him, was created. Speech is attributed to Him, in so far as the word which Moses heard, was produced and brought to existence by God in the same manner as He produced all His other works and creations. As we shall have to speak more fully on prophecy, we shall here merely show that speech is attributed to God in the same way as all other actions, which are similar to our own. When we are told that God addressed the Prophets and spoke to them, our minds are merely to receive a notion that there is a Divine knowledge to which the Prophets attain; we are to be impressed with the idea that the things which the Prophets communicate to us come from the Lord, and are not altogether the products of their own conceptions and ideas. (Friedlander translation)
The two terms, when applied to God, can only have one of the two last-mentioned significations, viz., he wills and he desires, or he thinks, and there is no difference whether the divine thought became known to man by means of an actual voice, or by one of those kinds of inspiration which I shall explain further on (II. chap. xxxviii.). We must not suppose that in speaking God employed voice or sound. or that He has a soul in which the thoughts reside, and that these thoughts are things superadded to His essence; but we ascribe and attribute to Him thoughts in the same manner as we ascribe to Him any other attributes. (Friedlander translation)
Whenever Moses went into the Tent of Meeting to speak with Him, he would hear the Voice speaking to him from above the kapporet that was on top of the Ark of the Testimony, between the two cherubim; thus He spoke to him (Numbers 7:89).
Not so with My servant Moses; he is trusted throughout My household. With him I speak mouth to mouth, plainly and not in riddles, and he gazes upon the likeness of the LORD(Numbers 12:7–8).
DIVREY YA'AKOV Chapter 3 verse 3: In shaar 3: chapter 11, he elaborates further that the whole world was formed by the command of Hashem, as recorded in parshat Bereishit. For each thing Hashem said that it would be, and so it was. The word of Hashem is a real spiritual entity with power to generate the entire existence of the world. He explains further that the spiritual reality of Hashem’s word is eternal and does not end with the days of creation. These continuously existing commands sustain and support everything in the world, like the living soul of all existence.
He also writes on the verse in Yeshayahu (40:5), “And all flesh together will see that the mouth of Hashem has spoken”. “The verse refers to the future time when Hashem’s supervision will be so clear, until we merit literally seeing with our physical eyes, how the word of Hashem is spread out over every thing sustaining it” - see the text inside for his full essay.
He adds that there was already a sample glimpse of this concept at Mt. Sinai at the giving of the Torah. This is referred to in the verse in Parshat Yitro (20:18), “And all the people saw the sounds” [The simple meaning here is the sounds which occurred uniquely at the Mt. Sinai event. But there are many ways and perceptions of the Torah, as is clear from Chazal, and there are many understandings to every verse given at Sinai. There are the simple meanings, and the meanings which are hinted to within the words, that which can be expounded from it, and the hidden secrets of it]. Here the sounds refer to the sounds of the word of Hashem at the time of the creation of the world, meaning that they merited seeing how this voice is the soul of the creation, sustaining it in its entirety. See how this is explained later in the chapter of the giving of the Torah at Mt. Sinai.
To me, the question is legitimate and there are a couple of possibilities:
G-d's control over the matter is absolute, He could create sound waves out of nothing, without a physical source.
G-d's control over people minds is also absolute. He could create a sense of sound in the heads of millions without creating sound waves in the first place.
G-d's control over people's memory could allow planting artificial memories in the heads of millions.
Either way, what's important is that according to the Bible, millions of Jews had this memory of G-d speaking to them.