Audible inside your head, or outside your head? If outside, i would say mental illness! Even when God spoke to Paul in Acts 9 on the road to Damascus, his fellow travellers did not hear the voice.
God spoke to me clearly when he saved me. He spoke to me when he called me to a Baptist church, when I had been out of fellowship for 2 years. He spoke to me in the early 1990's when he called me to seminary, giving me information I did not know. He spoke to me in 2005 when he called me to seminary the second time. It was the same voice, but in my head. And not intrusive thoughts.
I was unsaved when he spoke to me the first time. I can still hear it, it was so powerful. It was a voice you couod not deny! I was Pentecostal the second time he spoke to me, to call me to seminary. I was broken and hurting, sitting in a Baptist church when he spoke to me the next time, and what he told me came to pass! I was Baptist when he called me the second time to seminary.
I don't think the denomination has anything to do with it! God can speak to anyone he wants, when he wants!
Oh, and God speaks to me all the time in his Word. He also speaks to me when I pray, if I take the time to listen! But that is the "small, still voice." The kind that spoke to Elijah!
"To him the gatekeeper opens. The sheep hear his voice, and he calls his own sheep by name and leads them out. 4 When he has brought out all his own, he goes before them, and the sheep follow him, for they know his voice." John 10:3-4
Not sure how you can NOT hear the voice of the good Shepherd, if you are following him. Although I do know people who have not heard God's voice, who follow Christ by faith. I'm more of a doubting Thomas, it helps to hear his voice, especially when he saved me. I had to see, (well, hear) to believe. No, i have never seen God!
PS I work with the mentally ill. The Christians hear his voice, and they say it is different than audio hallucinations. The non Christians do not hear his voice. That makes sense to me!