If we read the Bible, we see that God spoke to and through many prophets. Some prophets also had visions. Did prophets hear an 'inner voice'? Did they perceive it as an audible voice?
God can communicate in different ways. I suspect the experience of Biblical prophets differed from prophet to prophet. God spoke with Moses as a man speaks with His friend, but other prophets would receive revelations through dreams, visions, and dark sayings.
If you talk with believers who have perceived God speaking or communicating with them, whether through the gift of prophecy or just through receiving direction, it can come in different ways. Some people just suddenly know something. This might fall more into the 'word of knowledge' category.
For example, if you are praying for someone and suddenly know some traumatic event, relationship problem, past sin, or mundane problem the person is praying about. I was praying for a man at a church retreat altar call I'd just met a couple of times at church and did not really know. He said I was praying out loud about the things he was praying for silently. I prayed for him to get another car. I did not know the backstory, but the wheel on his used car was making a strange noise when he drove it. While I was praying this, he was praying about his car. For me, I'd get either ideas about things to pray for that came to me. I had experienced that many times, and had people ask me how they knew things about them. I think of it as 'word of knowledge.'
Prophesying can come as a flow of words, so you might consider that to come through an 'inner voice.' In my own experience, it does seem like I get sentences that come to me, like a conversation, but not an external voice. I could not say what the voice sounds like. It is similar to how I think, insofar as I don't actually hear a voice with a certain tone when I think, but there are words. But I don't have people ask me, "How do you know that about me?" if I just think of someone and tell them.
I have only experienced this a few times that I can recall, but another way of getting some kind of message from the Lord is a picture in my mind's eye. I don't generally think in pictures. I tend to dream in unrealistic color and it doesn't look real. I would have great difficulty building something in my mind and turning it around. Some other people have very visual minds. So the picture in the mind's eye thing may stand out to me more, since my mind does not come up with these things all the time.
I consider the picture in the mind's eye thing to fall into the 'vision' category. Other visions are probably 3D experiences that look like real life, like you are there. An angel woke Peter up and led him out of the prison cell. But Peter thought he was in a vision. He could smell the smells, feel the air, and feel the ground beneath his feet, but he thought that was a vision. So I wonder what the animals in the sheet vision was like for him.
Sometimes an individual might get a picture in his mind that is a revelation for a particular person or a situation. Some people just flow in this. They pray for you and get these visions, which give them direction to pray or they get an interpretation, hopefully, as to what they mean.
I do not think we can say these things are limited to Israel or the church, since Balaam could hear God and even prophesied. But when it comes to dreams, we have examples of Gentiles having dreams given by God in the Old Testament. And there is a passage in Job which talks about how God warns men through dreams. Sometimes, dreams, like visions, can be had to interpret. Prophecies can also be hard to interpret. God told Moses that he would speak through visions, dreams, and dark sayings.
But not every 'inner voice', dream, or thought is from God. If someone asks 'Is the inner voice God speaking?' It depends on which voice. People have their own thoughts. Maybe demons can put thoughts in some people's minds. The Holy Spirit can also speak to people. Angels might be able to as well. They certain can in supernatural experiences where one sees the angel as we see in the Bible.