It is a library for mixing input MIDI and/or WAV files to output WAV files. To convert MIDI to WAV, [Sine]thesizer and FluidSynth are used.
Although modern DAWs have a lot more features, there is a strong reason to consider this minimalistic tool. A mixing project becomes more reproducible and more comprehensible if all steps are listed sequentially in a text file instead of being scattered over multiple GUIs (probably, with drop-down menus or nested screens).
To start with, there is a tutorial where this library is used for generating polyphonic audio from MIDI representation of a demo fugue. If something is not covered by the tutorial, try to read docstrings from the source code. These docstrings are informative enough.
To install a stable version, run:
pip install pymixerAbove command also installs sinethesizer Python package as a dependency, but it does not install fluidsynth (which is not a Python package). Please install it according to instructions from its official website.