Here's the thing, you downloaded this/these files from an SD card...who knows what all was included within that file, or what it could have tried to install or use to play it. Unfortunately, unless you can give us the file type or the information on what was downloaded from the file, we're not going to be able to help you much on figuring it out. We also don't know what kind of media players and tweaks you've done to your Mac, so again, without that information, what can you honestly expect any of us to tell you?
It's possible that the file not only came with the audio book but also an application to play the file with (something that may not function properly with Mavericks would be my guess as to why it was hidden). You could also check your activity monitor to see what kinds of functions are running and compare them to a normal running Mac environment.
You see, there are just so many different variables, none of which you've offered as information, that can cause different actions to happen...unfortunately, you've just been too vague or simply don't understand what it is you were DL'ing before you did it.