Q: It said God allowed Aaron up onto the mountain. Did this just happen once? Also, did all of Israel hear the 10 commandments spoken by God or were they too afraid and had Moses tell them?
A: Great questions. Aaron probably went up the mountain twice. Here in chapter 19, it records that God told Moses to bring Aaron, but it doesn’t actually say that happened. I’m guessing it’s implied that he did (this wouldn’t be the best time to disobey God’s orders..) Then in chapter 24, we see Aaron, his sons, and seventy of the elders ascend the mountain.
To your second question. Yes. All Israel heard God’s voice from the mountain. In fact, in verse 19, they were so terrified that they asked Moses to tell God to stop. I can’t imagine how powerful and forceful God’s voice coming from heaven was.
To explain further. God spoke only the Ten Commandments (verses 2-17) from heaven, and the laws which follow at the end of chapter 20 and into the next few chapters, he gave privately to Moses. Later we will see God write the words of verses 2-17 on stone tablets, and all the other laws were written on scrolls by Moses.