I've just barely started thinking about this, but a couple songs I love right now that I think will bring me comfort and inspiration in labor are
Wonderful by My Morning Jacket and Flower by Amos Lee. I expect some reggae will end up on my playlist too, probably old Van Morrison, maybe the Band; Rings by Mike Gordon and Leo Kottke makes me happy too.
I'm eager to work on this. Last time I didn't have an ipod yet and just had favorite CDs packed, but all I did was push once I got to the birth center, so we never put music on there. We probably had music on at home during labor, but I don't remember. I might end up making a few playlists this time - one for mellow early labor at home, one for soothing me during the hour long car ride to the birth center, and one for the the tougher parts of labor and/or after baby is out (not sure I'll actually want music when I'm really focused on birthing but maybe right after).
Looking forward to getting more ideas here.