“The man who hears God speak and can still inquire about the act corresponding, would simply show thereby that he actually has not heard God speak. We might, for example, hear Christian sermons preached and ask ourselves: What happens in virtue of the fact that this thing happens? To all these words what corresponds in reality? A question that most certainly needs raising! We might listen to Holy Scripture and hear only words, a man’s words, which we do or do not understand, but along with which invariably the corresponding event is still wanting. It is then sure that in the proclamation as in the Bible what we heard was not the Word of God. Had it been the Word of God, we would never have been looking around for God’s acts. The Word of God itself would have been the act. The Word of God needs no supplementing by the act. The Word of God is itself the act of God… The Word of God in the highest sense makes history.” (Church Dogmatics, I/1, p163)
The Word is the act.
Therefore preaching is practical.
A sort of ex opere operato view of preaching: it is God speaking whether it is received rightly or not, and it always achieves the purposes for which God sends it (though not always or automatically conveying grace)?
I think ex opere operato is a problem for anyone with a high view of preaching (which Barth certainly has). (Think of PT's "When God's word is taught, God's voice is heard" - could sound ex opere). But I see Barth trying to counteract that here when he speaks of the unaffected hearer not truly hearing the Word. More a doctrine of "real presence" than "ex opere".