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. Game designer Ben Esposito put them all together into a website that auto-plays them and lets you skip around between locations.
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: