I just listened to lecture 13 about the Washington Administration on my iPod a couple days ago and noticed the cut off at the end as well. I looked into it just now, and immediately noticed a discrepancy. If you look at the video\audio that you can play directly from the lecture page, both are 44:35 in length, and they do not stop abruptly at the end, But when you go to the main course page and download the whole course packaged into a ZIP file, lecture 13 is 52:09 long. I listened to the first few moments of both versions, and they seem to be different recordings.
I seem to recall someone saying that a couple of the lectures had to be redone at one point, but I don’t remember which ones. Maybe the audiobook download and the ZIP file that contains the whole course simply needs to be updated with the new audio recordings that are on the individual lecture pages?