modelinst.c is a simple Cmix instrument to read two input files in and mix them together to one outputfile. profile.c is linked with it and introduces it to cmix according to the rules in the Makefile. Read the manual pages on usersub, and sfheader for more information. Also see the unit generators (filters, combs, reverbs), etc in /usr/users/paul/cmix/lib for more information. There are manual pages on most of these. These are stored in the library cgenlib.a, and this is accessed in the Makefile rules. The important protocols in this example are setnote(), GETIN(), ADDOUT() and endnote(). Also the arguments passed to the subroutine itself, and the way it is introduced to Cmix. The rest is up to you.
These are the contents of the former NiCE NeXT User Group NeXTSTEP/OpenStep software archive, currently hosted by Netfuture.ch.