In SCI, each MIDI track in a sound resource is enabled or disabled based on what driver is used. You can change the sound driver in the combobox on the left and see what tracks are enabled. But I think by default when you import a MIDI, it just enables all tracks for every driver? I forget.
Anyway, use an existing game to find a driver that works on your setup (whether that's DOSBox or ScummVM?)... you can set a sound driver in the resource.cfg, or run the tool that lets you choose from a menu. If you get a sound working in an existing game, it should work in your custom game too - as long as you make sure that sound resource has tracks enabled for that sound driver (or just enable all tracks for all sound drivers?).