How does one playback MIDI files on Mac OS X?

In a terminal window:

brew install timidity; timidity yourfile.mid

If you don't have brew, get it here: https://brew.sh/

The recent VLC 3.0.0 has returned MIDI support and works on macOS.

Aria Maestosa looks a bit dodgy, couldn't open any of the midi files I downloaded... A better option is probably to install Quicktime 7, which supports MIDI, and still runs in Yosemite, check out http://osxdaily.com/2014/07/20/run-quicktime-player-7-in-mac-os-x/

I also discovered https://github.com/SamusAranX/MinimalMIDIPlayer recently. It is still very basic but the fact you can use custom sound fonts is promising. Hopefully the developer adds a global soundfont setting soon so you don't have to rename your soundfont to match the midi file you want to play.