I have a student in my current class who told me that she simply couldn't understand the audio recordings we're using in class. That's not so unusual, but after working with her for a bit in my office, I came to believe she has an unusual disjunction between her reading ability and listening ability. So I whipped out Audacity and started playing around with the files.
There is an option to change the tempo (i.e., make it slower without changing the pitch), but it introduces an odd distortion to the voice. Instead, I found it better change the speed and then boost the pitch back up. To do this, you begin by opening your file (after you've downloaded and installed Audacity). Step 2 is to select the portion of the file you'd like to slow down. In most cases this will be the whole recording. Step 3 is to click on Effect from the menu bar, and choose change speed, perhaps by 33%. Once this completed, you need to once more click on Effect and then choose change pitch. I found the pitch needed to be boosted slightly more than the amount of the speed reduction (if you can compare those two things). So, a speed change of -33% would require a pitch boost of roughly 43%.
Here's are some samples: