The "default" (ie. Roland MT-32) music sounded and still sounds marvellous... as long as you either have a genuine Roland MT-32 (or similar) device, or if you use a good MT-32 emulator, ie. Munt.

The reason it sounds so bad without those is because Windows General MIDI driver is trying to play the MT-32 MIDI messages it gets from the game/DOSBox, thinking they are General MIDI messages.

So, the game sends out a MIDI message that the MIDI driver/device should play e.g. instrument #3 (e.g. a piano sound), but Windows GM driver plays some horn instead because that's what instrument #3 is in General MIDI specifications, etc. (a made up example, I don't know how exactly the MT-32 and GM MIDI specs go).

That's the reason why by default Roland MT-32 music sounds so bad and plain wrong on General MIDI sound cards, and vice versa. Some GM cards had a "MT-32 mode" which (I think) would rearrange the instrument placements to be better suited for playing MT-32 music, but they wouldn't work too well either if the game was changing the MT-32 default settings.