God would have to voice his voice every time a new life was brought into the world. Not to mention the amount of dough (cash) that would flowing through into the hands of Psychiatrists, Pharmacists and mental institutes.
The answer is he already has, through the bible.
Every time you read his word, his voice comes to life. If written word isn't satisfying enough, what makes people think that a miracle would change their mind? I would say the documents of the bible are a miracle, based on the fact that we have them readily available. Thousands of years of history that has managed to survive without being destroyed by the civilizations which ruled over Israel. You gotta admit, thats pretty impressive if you look at the history behind the scripture.
All through out the bible, miracles were preformed and people still rejected and denied God, including those that witnessed them first hand. The issue isn't so much to do with our senses or evidence (though both are important), but with our heart and ability to submit.
The greatest miracle was the birth, the death and the resurrection of Christ. Unfortunately, even that isn't good enough for allot of people, including those that witnessed it and prosecuted him in the first place.
I will say this, God doesn't do miracles to appeal to us or for our entertainment, God doesn't need to prove anything to anyone. Imagine if people starting saying "I can make God do whatever I ask him to do", in other words that's saying "I am above God, he is my servant". That's pride, and God opposes all pride. Unfortunately, some (if not many) people who claim to be Christians often carry this attitude.
Psalms 138:6
Proverbs 3:34
Proverbs 16:19
James 4:6
1 Peter 5:5
And thats only to name a few.
God does miracles to glorify himself. Ultimately what God sees as fit, is beneficial for us, whether we recognize it now or later. Though mind you, his perspective and our perspective are completely different. He's perfect, we obviously aren't, so our understanding and recognition of things is inhibited (flawed). We will never come to understand why God chooses to do or not do, until he says so and there are many things that God has not told us, most likely for the better.