I'm sure that I'm not the only person to have discovered this, but here you go:
Slowly pull the headphone jack out while you are listening to a favorite song and at some point you will get a "ghosting" effect. It sounds like you are hearing the song as if you were standing outside the doors to a concert venue and were hearing the sound check.
I have a IPod 2nd Gen, so I'm not sure if this will work on all versions or any other MP3 player. I will be getting a IPod 5th Gen later this week and will try it then.
While this does sound good and even great on some songs, others not so much. Great song to try it on is Steve Perry's "Missing You" DON'T try it on is Steve Perry's "I Stand Alone", something about the way that song is layered makes it horrible to listen to that way, even though it is one of my favorite songs.
Anyway hope some of you find some enjoyment out of this little trick.
Rich the Layered Guy