The Secret of Monkey Island doesn't have a setup utility or config file. The game attempts to auto-detect your hardware and use the best available option. You can start the game with command line parameters to override the auto-detection. What sound modes are available to you depends on which version of the game you have (EGA floppy, VGA floppy, VGA CD version). Type "MONKEY ?" at the DOS prompt to get a list of supported parameters.

I don't think the game ever had a pure SoundBlaster option (I'm not sure if SoundBlaster cards with dual OPL2 or OPL3 even existed when the game was released). The original SoundBlaster was basically the same as the AdLib anyway as far as music is concerned. Some versions support GameBlaster cards (run the game with MONKEY G to use this), but I think AdLib sounds much better than that.

The only thing close to GeneralMIDI would be the Roland mode. But I don't know how to use that for GeneralMIDI in DOSBox, and some SCUMM games also require additional files for the Roland mode.

You're probably better off playing the game in ScummVM. It's much easer to get GeneralMIDI music via ScummVM. Once you have added the game to your ScummVM games list, select "Edit Game..." and pick a MIDI device for the music in the "Audio" tab. I would also recommend enabling the "Mixed AdLib/MIDI mode" setting in the MIDI tab, otherwise you would miss out on most of the sound effects.

Set up Munt (a very good MT-32 emulator) on your machine and run the game with "MONKEY.EXE R" to use MT-32. Or use DOSBox-ECE which has Munt built-in and thus is much easier to set up:

https://dosboxece.yesterplay.net/en

In both cases, you will still need to get the two required MT-32 ROMs for it to work.

This is how it should sound like if you've set up everything correctly:

https://www.youtube.com/watch?v=3BAcXzljDy0