I managed to setup RetroArch (using dosbox pure) so that it'll use garageband as an external midi sound canvas. But there's a problem: when I go into the dos setup for duke nukem 3D to general midi or sound canvas and then I do a music soundcard, it plays the whole audio through one instrument (e.g. piano) - I can also import the duke nukem 3D grabbage theme (to get all the instruments loaded up) but when I set every instrument to play together, they play the whole thing as if it was one midi track just copied and pasted into many instruments which means that when the drums should only play the whole lot of the instruments play the song.

Does anyone know how I can setup garageband/ the mac's midi studio so that I get an output as a multi track midi instead of one track/channel midi? Or would I need a plugin for garageband for that to work properly?