Daniela, I agree. Looking at a static screen while someone invisible is talking about it is worse than listening to a lecture. At least during a lecture you're looking at a living/animated person who is using their body language/intonation to give more life to their message (if a lecturer has good presentation skills, of course).
Even if you're listening to a tour guide while admiring art/architecture, you don't really stare at that art for as long as the guide is talking - you phase out/in, look at the guide, maybe wander off to look at something else if the guide takes too long to describe all the intricacies of the artist's style. Not to mention that art/architecture is usually more complex than an e-learning slide and you'd usually move your eyes and switch attention between different details. In a way, you're adding 'animation' to the 'audio'.
Listening to audio while looking at nothing and not being able to escape is a highly unnatural situation for a human being! :)