I would agree that when God spoke, He is not doing so in the same material way that humans do, by making vibrations creating waves in the air. When God first spoke, there was no air to vibrate. Formally, speaking refers to manifesting your thoughts or your mind outside of your own mind and outside of yourself. So, when God, spoke, it meant that He reached outside himself (in a sense) and expressed His mind in His creation. His creation are His words in that they reflect His mind.