You'd need to include average length of the sermon to determine if the size is somehow inflated by your original program.
What program were they originally created under?
What OS do you have?
Have you tried Audacity?
The programs were originally created in iTunes. I have looked at Audacity, but a quick multiprocessing didn't seem possible. I actually figured out an easy way to do it in iTunes by selecting the tracks to convert, then right-clicking and selecting Make mp3 version. After some more searching, I found some recommended settings here:
http://www.church123.com/church_website_online_sermons.htm
The files are all about an hour in length. A 100 mb file is a bit too big for simple speaking!