In the past God spoke to our forefathers through the prophets in many portions and in many ways.
One question that has been rolling around in my mind for a while is whether or not it is appropriate to call God an "author" of Scripture. After all, God didn't actually put the pen to the paper. Furthermore, Scripture never calls God an "author." There is talk of inspiration, etc., but I don't believe the term "author" is ever used.
This passage in Hebrews opens up a labyrinth of hermeneutical possibilities. On the one hand it clearly points out the common sense notion that we have of the multiple authors and voices in Scripture. The written Bible is a composition of a variety of expressions and perspectives. It is diverse and far from monolithic.
In some cases God revealed a specific word through a prophet, presumably in a verbal way. These words were then repeated to the nation(s), and later written down as a testimony of God's spoken Word. In other passages God simply seems to be revealing through a poetic expression via the Psalms. Or he may use the historical account as a means of speaking Scripture. And, in a very rare case, God writes directly upon the stone and reveals his decalogue to Moses on Mount Sinai.
There is incredible diversity in Scripture, which raises the question of how God remains the "author" of all of these "various ways" spoken of in Hebrews 1:1. To call God an "author" in this sense seems vague and uninformative. And yet if God is actually speaking in all of these diverse scenarios then there is a sense in which each and every text is a creation of God's authority.
So, perhaps God did not speak in one, particular way. Perhaps the process of God's inspiritation of Scripture is as diverse as the Scriptures themselves.