First of all, someone that is "hearing voices" due to a mental illness, is not just hearing a voice once in a great while, but often. And they probably exhibit other symptoms like paranoia and other delusions.
If God spoke to someone in an audible voice, or an angel, it would not contradict the word of God. It would probably also confirm some other "sign" that person had already had like another person telling them the same thing etc
Is that a yes?
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Hebrews 13:23 Know ye that our brother Timothy is set at liberty