for this situation i recommend leaving "Sounds and Audio Devices" midi selection on its default.. whatever your soundcard normally offers for general midi.
in DOSBOX at the prompt type mixer /listmidi. remember the number listed for "MT-32 Synth Emulator". Then in the dosbox.conf file under [midi] change the line 'midiconfig=' to whatever number MT-32 was listed as.
This way all your modern programs expecting general midi default to it and your old dos games expecting MT-32 sound.. so... much... better... I find this to be the most elegant solution.