God communicates Himself to the world not as He is in His essential nature (which is infinite and as such beyond our comprehension in any case), but as we are capable of receiving. This means He provides a visible appearance for our eyes, a sound for our ears, and a body of laws through a mediator (Moses, to the Jews). Calvin says that this is God speaking to us as we speak to infants and children, not as we really are but as they can understand us. God even in His communication condescends to us.
We must not, however, confuse this physical appearance with a strictly natural religion. The point of these physical and temporal experiences was not that we should look for material prosperity, but that our eyes should be lifted beyond the physical to heaven and to the worship of God Himself. But God did not reveal this all at once, choosing instead to gradually, over time, reveal the nature of true religion and proper worship in history.
So when God speaks to angels, it is not as He does to us when He speaks to mankind's physical ears. Rather it is the communion of spiritual beings, His will transmitting itself to their minds, and their minds translating instantly to will, delight, and action.