I had the same problem. Assuming they are mp3 format. You need to select them and export them as the... whatever apple's audio file is... AAC I think. So, you'll "right click" the mp3 in iTunes, and select the option about creating an AAC version or export as AAC, whatever option the menu gives you. See if that helps.
Regarding the Goljan audio, he is the man. However, keep in mind that:
1. The versions going around the web are about 10 years old now (from what I've heard).
2. They are great at giving you an overall idea of things, but they lack some of the detail your school and professors may want you to know.