Not a sound guy, but here's my thoughts. I had trouble finding anything that would convert sound to text, except for Dragon Naturally Speaking1 It is easier to go from text to sound than from sound to text.
I would download mp3s and transfer them on to a CD or mp3 player, and then connect the line out, if available (or headphone out, if not - not sure if this is safe) into line-in on sound card. You might need to set options on Dragon to 'listen' to line-in not microphone. Then you ought to be able to get Dragon to work somehow (not very helpful, I'm afraid). There must be other speech recognition software around (XP comes with some elementary stuff, which might be useable?) but whatever you use you will need to learn how to use the software whether its Dragon or not; normally it needs training to understand the person, so you might need to reset it for different speakers.
One thinks you ought to be able to feed an mp3 into some software directly, but I don't know any, and none was obvious on the web.
Sorry, this wasn't really a perfect answer, but I hope it helps a little, and it will put it back to the top of the entries. There ought to be at least one sound engineer that listens to good sermons on SI that can give you a better answer?1?
Mike
PS We could start a whole new rabbit trail on whether its right for Christians to use Dragon software, but better not. 8-)