[quoting Tripp, 21 Oct 96.a] From a Darwinian viewpoint my results are a little surprising. Human have a genetically derived ability to hear sounds as words and sentences and assign meanings to these. This ability presumably evolved about 200,000 years ago. It took a long time before humans realized they could write down sounds and words and read them. In many ways, reading and writing are extremely unnatural, in the sense that they are inventions, not genetic mutations. So it is surprising that the unnatural medium is "better" than the natural medium.
It may be that the visual cortex is more "powerful" than the audio processing areas of the brain or something, but this is pure speculation, and I have no special insights.
I would speculate that it has to do with the amount of control associated with the written text. That is, students' can control the speed of their reading, they can focus more on a section they don't understand, and skim over sections they do, etc.--more "metacogntive" awareness. Even if they would have been allowed to control the speed of the tape, they probably still wouldn't have had the control they had when reading.
I am not implying that this explanation invalidates in any way the principal purpose of your paper--just some thoughts on the advantages of written word in the context of your experiment. By the way, I don't have a reference handy, but there is evidence that good readers change speeds more often (are more metacogntively aware) than poor readers.