You know, people aren't doing enough modifications to baby stuff. I did my best with my Bugaboo cup holder a while back (which is still working well for me by the way), but the best one I've seen yet I saw on Daddy Types this morning. Some resourceful father hooked up some JBL On Tour portable speakers and an iPod full of lullabies to his kid's crib. My baby doesn't need any help getting to sleep, but I love the idea. The only thing I can see that's missing is a timer so the music stops eventually - but I suppose limited playlists would solve that problem. A remote so you could turn it off once your baby fell asleep (like a mobile) would be sweet too.
Anyway, get all the particulars at Daddy Types: MP3 Crib.