Listening is passive. It is the lowest, least-efficient,
least-effective form of learning. That means lectures are the lowest,
least-efficient, least-effective form of learning. Listening alone
requires very little brain effort on the learner’s part (and that goes
for reading lecture-like texts as well), so listening to learn is often
like watching someone lift weights in order to get in shape.
Thinking along these lines, though, we are assuming the goal of preaching is learning. Even though the terms "teaching" and "preaching" are regularly used synonymously, I think we could stand to make a distinction between the two. The goal of teaching is learning (especially learning to do something), but the goal of preaching is probably something different. Preaching is proclamation, in word and deed, that the kingdom has come.
Anyway, whatever you call what the pastor does when he talks for 30-40 minutes on Sunday morning, the realization that simple listening is the least-efficient form of learning should affect how we go about it.