I don't know whether you can play mod files in ds6 but you can do this with MIDAS:
http://www.s2.org/midas/

Is it possible to use .mod and .s3m-files in my game with DirectSound? I mean with a simple command.

If not, where can I find a library that can play mod,s3m-music?

My friend told me there's something called "ModPlug SDK" that plays almost every MOD-Format (including .XM and .IT).
I think it can be found at http://www.modplug.com

See you...