How did they hear Him? Perhaps each one in a different way, but hear they did and when they obeyed God they were then on His side. I cannot speak for everyone else, but usually I don't hear a physically audible voice from God. I say usually, because sometimes He puts His words in the mouth of the man and I do hear them with the natural ear. Normally the hearing is only in my heart. A very few times long ago I heard Him through a dream.
I think they all heard in the same way. The Voice of God is not heard as a "physically audible voice" that comes out of thin air unless someone is being taught to recognize it as Samuel did as a young boy. He thought Eli calling him. The person needs to learn to the difference between a physical voice and a spiritual voice so he won't be confused.
The person who does not "see" anything but who "hears" only a voice has some problems at first. When God wishes to establish a "line of communication" with that person so he knows for certain that it's the Voice of God and not something out of his own imagination, He will hear the Voice first in a dream. All the distractions of daily life are shut out then, too. Thus Moses wrote of two methods God chooses at first so the person gains confidence that he's hearing right.
Numbers 12:6 And he said, Hear now my words: If there be a prophet among you, I the Lord will make myself known unto him in a vision, and will speak unto him in a dream.
I found it remarkable when it happened to me. I didn't know about that verse in the Bible when it happened. I had a dream and heard only a voice and didn't see a thing.
If the Voice of God does manifest to people with impure hearts, it cannot get through completely. At Sinai, Israel could "hear" on ten things with intelligence. The rest they heard as a threatening loud trumpet sound. Only Moses heard all the intelligence in the Voice's message. Israel wanted to follow God but they didn't want to hear the Real Word, so Moses wrote things down for them in the "Written Word." Today when Jews read the Torah, they strain trying to hear the Voice. They know they have hardness of heart that is preventing them from understanding what God really wants them to know. They know they need to circumcise their hearts to gain the proper understanding of the spiritual truths behind "human words" that Moses gave them.
David wrote about the Voice of God and wanted the people in his generation to hear it.
Psalm 95:7 For he is our God; and we are the people of his pasture, and the sheep of his hand. To day if ye will hear his voice,
8 Harden not your heart, as in the provocation, and as in the day of temptation in the wilderness:
9 When your fathers tempted me, proved me, and saw my work.
10 Forty years long was I grieved with this generation, and said, It is a people that do err in their heart, and they have not known my ways:
11 Unto whom I sware in my wrath that they should not enter into my rest.
I don't think the Bible may be enough, really, to enter the eternal rest. We may need to get beyond the Bible and to hear for ourselves; but if we cannot hear for ourselves, it is good to rely on the words the prophets wrote for us or the words other men of God can speak to us.
I think it something of a mistake then to call the Bible the "Word of God." It is human words that prophets wrote down when they heard the Living Word of God. Human words have their limitations. Human language is not perfect.
I've also read that "age" can block the hearing of spiritual voices. Prophecy fades away? All we may be left with is charity. I think that's because the body may be too distracting when we get old.