MusiXTeX, created by Daniel Taupin, Ross Mitchell, and Andreas Egler, is a set of macros and fonts that enables TeX to typeset music. To use it, at a minimum you must copy the macro and font files to the right directories in your particular TeX file structure, have the binary musixflx available to run, and have a TeX input file containing the proper commands. It's a three pass system: TeX, musixflx, then TeX again.
To simplify inputting instrumental music there is the preprocessor PMX by Don Simons, which creates a TeX input file. For vocal music, there is M-Tx, a preprocessor for PMX by Dirk Laurie. It provides easy access to the functionality of the TeX macro package for lyrics called musixlyr, by Rainer Dunker.