number - How was Moses able to speak to at least 600,000 people?

The book of Exodus says that Israelites had 600,000 men of fighting age. And often it has paragraphs like this:

Exodus 35:4 (JPS):

And Moses spoke unto all the congregation of the children of Israel, saying: 'This is the thing which the LORD commanded, saying:

There are many, many more examples like this. Sometimes, right after Moses says something, events happen that affect the whole community, suggesting that he was speaking everyone atthe same time:

Numbers 16 (JPS):

31 And it came to pass, as he made an end of speaking all these words, that the ground did cleave asunder that was under them. 32 And the earth opened her mouth and swallowed them up, and their households, and all the men that appertained unto Korah, and all their goods. 33 So they, and all that appertained to them, went down alive into the pit; and the earth closed upon them, and they perished from among the assembly.

So, how was a community of at least 600,000 males able to hear Moses when he addressed them? That is thirty times more people than would fill Madison Square Garden.

And just a follow up question: if Moses actually spoke to the leaders, which passed things down to their respective tribes and groups, then doesn't Judaism actually become a religion handed down by a small group of people? At this point, the best argument for its authority is that God specifically set up this structure and said it was the best.