Explaining the voice of God is not easy. Most people assume God's voice will be an audible voice. That's not normal. Most people's experience with the voice of God comes as simply knowing something. The apostle Paul once said he was pressed by the spirit to preach the word of God (Acts 18:5). He just knew he had to. That was the spirit of God directing him to do something.
Elijah heard what he called the still, small voice.
1st Kings 19:12 KJV And after the earthquake a fire; but the LORD was not in the fire: and after the fire a still small voice.
This is likely the voice that the prophets interacted with. It's as if you hear a whisper in your own mind. God can literally drop His thoughts into your mind and you assume that they are your thoughts. It's not easy to wrap your mind around that notion. You need faith to believe when you hear a whisper in your own mind. The prophets did. Check out this word through Zechariah.
Zechariah 7:12 KJV Yea, they made their hearts as an adamant stone, lest they should hear the law, and the words which the LORD of hosts hath sent in his spirit by the former prophets: therefore came a great wrath from the LORD of hosts.
Looking past the doom and gloom in this verse, we see that the Lord sent words IN HIS SPIRIT. That's unusual wording. It doesn't say that God spoke to them, though that's what He did. It says He sent them words in His Spirit. You have to get that.
I'm not saying every random thought is from God. That's where discernment and testing the spirits comes into play (1st John 4:1). All I'm saying is that it's difficult to hear the voice of God if you're not paying attention. His voice comes with volume control. He can make it the big, booming voice from the sky. He can make it a dream or vision (Acts 2:17, Isa 2:1). He can also make it a still, small voice.