This is simply not true. Unregens can hear the voice of God as clearly as we can. How can a person who doesn't believe in Christ hear and learn of the father if it is not the holy spirit teaching him?
No man can come to me, except the Father which hath sent me draw him: and I will raise him up at the last day. It is written in the prophets, And they shall be all taught of God. Every man therefore that hath heard, and hath learned of the Father, cometh unto me. John 6:44-45
And they can obey the holy spirit. How else are they producing fruits of the spirit? We've all seen some unbelievers doing these things
But the fruit of the Spirit is love, joy, peace, longsuffering, gentleness, goodness, faith, Meekness, temperance: against such there is no law. Galatians 5:22-23
So maybe they are following his spirit, but just haven't yet had their eyes opened to the full truth. I'm actually speaking from experience. When God opened my eyes I knew it was the truth because it was the voice I had been following and longing for.