I think Bible is the word of God because God has told me that.
Other people have heard God speak to them. Ever hear of Vince Li? God spoke to him, too.
God spoke to Terry Mark Mangum, telling him to "carry out a code of retribution" and kill a homosexual flight attendant.
George Bush heard God, and believed that the wars in the Middle East were a fulfillment of Biblical prophesy.
If you hear a voice talking to you, and no one is there, that is a sign of schizophrenia, not God. You should get psychiatric help right away, before that voice tells you to do something like what Vince Li did.