If you want to make procedural music, generally you're going to need individual note sounds (for whatever instrument) which you can string together. Is there somewhere i can get these for free, preferably usable for commercial purposes?
the best choice is to go to your favourite music school and sample the note yourself. this way, you are sure your sample will stay unencumbered with copyrights, distribution rights, etc... however, this is tedious.
you can also approximate the sound of an instrument using good old techniques like additive synthesis, and generate the sound using some set of VCO/VCF and filters. such sounds have been used for so many years, they sound quite vintage, but are relatively easy and inexpensive to generate.
i will finally add that your best option would be to generate notes instead of sounds, and let the specialized hardware do the sound generation according to your notes. there exists plenty (i mean really really a lot) of sound libraries that musicians using computer already own or buy. those libraries are generally driven by a MIDI instrument, be it a simple virtual MIDI port software on a host computer or a full-blown VST plug-in hardware rack. keep it simple, generate some MIDI notes, and users will bring the sound themselves.