Change the file format of a song in iTunes Library. Example is AIFF to MP3. The text-to-speech programme we use to make voiceovers for presentations and talking stories saves the sound files as AIFF. We often end up with a narrated PowerPoint too big to share by email. MP3 is smaller but you still have to watch out if you want to email it. Making a movie of the slideshow can result in much smaller file size. See other tutorials here.