As far as I can tell, this is completely impossible. On 10.8.4. I am trying to play a sample MIDI file off of Wikipedia.
Quicktime v 10.2 fails to open a MIDI file with error message
The movie “Ii-V-I_turnaround_in_C_four-part_harmony.mid” can’t be opened because the file is not in a format that QuickTime Player understands.
VLC is also unhappy:
No suitable decoder module VLC does not support the audio or video format "MIDI". Unfortunately there is no way for you to fix this.
AFAICT I need to go find some random obscure MIDI player and download it for Mac, which seems extremely strange given that a site as large as Wikipedia uses them freely and assumes they "just work" on Mac. What am I missing?