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(All quotations from the King James Version unless stated otherwise).
As far as I am aware the Bible doesn’t say that Moses was heard in the desert by one million people. This is a supposition on your part. Initially God himself (via an angel) spoke to Israel from Mount Sinai and we might expect that literally ‘all the people’ could hear the words of God.
Exodus 19:16 And it came to pass on the third day in the morning, that there were thunders and lightnings, and a thick cloud upon the mount, and the voice of the trumpet exceeding loud; so that all the people that was in the camp trembled. 17 And Moses brought forth the people out of the camp to meet with God; and they stood at the nether part of the mount. 18 And mount Sinai was altogether on a smoke, because the LORD descended upon it in fire: and the smoke thereof ascended as the smoke of a furnace, and the whole mount quaked greatly. 19 And when the voice of the trumpet sounded long, and waxed louder and louder, Moses spake, and God answered him by a voice. 20 And the LORD came down upon mount Sinai, on the top of the mount: and the LORD called Moses up to the top of the mount; and Moses went up. 21 And the LORD said unto Moses, Go down, charge the people, lest they break through unto the LORD to gaze, and many of them perish. 22 And let the priests also, which come near to the LORD, sanctify themselves, lest the LORD break forth upon them. 23 And Moses said unto the LORD, The people cannot come up to mount Sinai: for thou chargedst us, saying, Set bounds about the mount, and sanctify it. 24 And the LORD said unto him, Away, get thee down, and thou shalt come up, thou, and Aaron with thee: but let not the priests and the people break through to come up unto the LORD, lest he break forth upon them. 25 So Moses went down unto the people, and spake unto them.
Exodus 20:1 And God spake all these words, saying, 2 I am the LORD thy God, which have brought thee out of the land of Egypt, out of the house of bondage. 3 Thou shalt have no other gods before me. 4 Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth: 5 Thou shalt not bow down thyself to them, nor serve them: for I the LORD thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me; 6 And shewing mercy unto thousands of them that love me, and keep my commandments. 7 Thou shalt not take the name of the LORD thy God in vain; for the LORD will not hold him guiltless that taketh his name in vain. 8 Remember the sabbath day, to keep it holy. 9 Six days shalt thou labour, and do all thy work: 10 But the seventh day is the sabbath of the LORD thy God: in it thou shalt not do any work, thou, nor thy son, nor thy daughter, thy manservant, nor thy maidservant, nor thy cattle, nor thy stranger that is within thy gates: 11 For in six days the LORD made heaven and earth, the sea, and all that in them is, and rested the seventh day: wherefore the LORD blessed the sabbath day, and hallowed it.
12 Honour thy father and thy mother: that thy days may be long upon the land which the LORD thy God giveth thee. 13 Thou shalt not kill. 14 Thou shalt not commit adultery. 15 Thou shalt not steal. 16 Thou shalt not bear false witness against thy neighbour. 17 Thou shalt not covet thy neighbour’s house, thou shalt not covet thy neighbour’s wife, nor his manservant, nor his maidservant, nor his ox, nor his ass, nor any thing that is thy neighbour’s.
At this point the people (surely meaning the people’s representatives) requested that God would no longer speak to them.
18 And all the people saw the thunderings, and the lightnings, and the noise of the trumpet, and the mountain smoking: and when the people saw it, they removed, and stood afar off. 19 And they said unto Moses, Speak thou with us, and we will hear: but let not God speak with us, lest we die.
After this, Moses spoke God’s words to the people. It seems reasonable to suppose that those who couldn’t hear him would have his words relayed to them by those who could. In addition Moses wrote all the words in a book (Hebrew ‘cepher’ – writing).
Exodus 24:7 And he took the book of the covenant, and read in the audience of the people: and they said, All that the LORD hath said will we do, and be obedient.
The book was given to the priests whose job it was to teach the people. In this way Moses’ words became available to the following generations and in these last days, to us.
Jesus validated Moses’ writings.
Luke 16:31 And he said unto him, If they hear not Moses and the prophets, neither will they be persuaded, though one rose from the dead.
The writer to the Hebrews seemingly has no problem with the mechanics of Moses speaking to all the people:
Hebrews 9:19 For when Moses had spoken every precept to all the people according to the law, he took the blood of calves and of goats, with water, and scarlet wool, and hyssop, and sprinkled both the book, and all the people,
I don’t believe it was necessary for literally ‘all the people’ to hear Moses speaking and a miracle wasn’t required. Certainly if ‘all the people’ could hear Moses, as you suggest, it would be a miracle.
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I hope you have found this helpful.
God bless you,