abc2midi seems to use its own sound mixer, so the problem does not arise at all

abc2midi will just be handling the conversion. A program like ABCExplorer or EasyABC (or even my own folkplay which is not an abc editor but uses abc for internal storage of songs) thing... will call abc2midi to perform the conversion and write the midi file and then use some means (built in or external program) to play the midi.

EasyABC uses a python midi module for this. folkplay uses the javax.sound.midi package but I'm not sure what AbcExplorer does.