Good day,
i have this same problem. Use retroarch my first time, load this both core: bsnes_accuracy_libretro.dll and bsnes_balanced_libretro.dll.
Have this "sgb.boot.rom (1kb)" file in my retroarch system folder. And the Super Game Boy 2 (Japan).sfc rom (512kb).
-edit-
okay, try
-L "cores\bsnes_balanced_libretro.dll" "C:\Users\Karsten\Desktop\LaunchBox\Emulator\RetroArch\Super Game Boy 2 (Japan).sfc" --subsystem sgb
and its work. BUT graphic and sound issues. Framerate?
VIDEO:
https://videobin.org/+ltr/q6k.html
I have nothing changed under the audio settings. use the retroarch default settings.