What module type files can be played with the SDL_Mixer object?

And is there a way that I could improve the quality of the audio that is mixed the SDL_Mixer?

I've noticed that the demos that come with it(Oxygen, etc) playback modules(ScreamTracker 3's S3M, FastTracker's XM and Impulse Tracker's IT, etc), but the quality could be better. I do not intend to use BASS or FMOD so please do not suggest them. Was the original C/C++version of SDL not open source?