I tried to upload an mp4 file as part of an imported lesson and it told me that I can only use mp3. Is there any way around this?

Our player actually only handles MP3 files, so you’ll have to convert them to MP3.

How would I go about doing that? The files are from a CD that I bought off of iTunes.

Here you go: Convert a song to a different file format with the Apple Music app or iTunes for Windows - Apple Support

Let me know if you have any more questions!