If you're trying to learn a new song and the chord changes or notes are just going by too fast, you're in the same position as the person who's trying to learn a foreign language. To slow things down without changing the pitch, you can use a commercial application such as BIAS Peak, or you can go the shareware route with something like the Amazing Slow Downer. But there's a new option you probably haven't thought of: Turn your files into an audiobook, then listen using newer versions of iTunes or iPods, which allow you to set the rate an audiobook is read without changing pitch. To do this, first convert the audio to AAC format (see below), then change the file type from .m4a to .m4b using the Finder.

Converting audio formats requires going into the iTunes preferences, then in the Advanced/Importing tabs, then select "Import Using: AAC Encoder" from the drop-down menu. Finally, select the track in the iTunes browser, then choose Advanced > Convert Selection to AAC.