It sounds like a silly idea, but put on headphones and listen to a minute of giant axes gently swinging in Sen's Fortress and tell me you don't feel more relaxed. YouTube user eroticfishcake recorded a collection of ambient sounds in Dark Souls, along with some rather pretty shots of the areas the sounds are from.
You can find all the clips on the site, but here are two samples:
Outside the entrance to Sen's Fortress, there are some nice bird songs:
For a more grim feel, the crying pisacas from the Duke's Archives: