Within the "projects" directory there are a couple of example projects that you might want to look at.
The examples include code for:
YM usage
AYFX usage
Vectorlist / Animation usage
Arkos Player usage
Usage of "S" sources from within C
usage of the old "3d" routines of mine
usage of BIOS functions
how to "replace" BIOS functions with "pure" C functions
(this is not possible for cycle exact timing like e.g.PrintStr though)
And probably many other things.