So here's a question that bugs me: I'm reading that DOSbox does not emulate MIDI hardware, it just passes MIDI info to whatever external synth device you have. And that in Windows there's just one General Midi wavetable. So how come that if I go to sound setup for any DOS game I get a totally different sound on each of the devices listed, including the mythical Roland (which also usually sounds the best). I don't have any synthesizer hardware, it's just me playing a game in DOSbox on Windows with an average onboard soundcard.
What is your question exactly?
For old DOS games there are just 4 (or 6) main options (Adlib, MT-32, Gravis Ultrasound and General MIDI compliant cards):
DOSBox emulation works perfectly fine, nothing can beat it except for native Adlib Gold card.
2. Roland MT-32
(also CM-32L, CM-64 and LAPC-I)
It was the best music option in early nineties, it predates General MIDI that's why they don't mix together well.
DOSBox don't have emulation for that but you can connect real thing into your modern PC and use it.
But the best thing would be to get respective ROMs: MT32_PCM.ROM, MT32_CONTROL.ROM, CM32L_PCM.ROM, CM32L_CONTROL.ROM (they are illegal for distribution unless you make a copy out of your own disc). And then install latest MUNT for near perfect emulation. https://sourceforge.net/projects/munt
/ Please note that this will add additional MIDI device
to your PC so you need to change it before you start playing specific MT32 friendly games.
3. Gravis Ultrasound
DOSBox can work with it just fine but you need to get additional files and put them into your DOSBox folder properly. It worked for me, some games like Doom 1 would sound much better than with General MIDI (Roland Sound Canvas).
This will NOT add additional MIDI device, but if we are talking about GOG games then you'll have to tweak separate DOSBox folders respectively and change the music device in setup of course (some GOG games like Wing Commander 1 don't have a way to setup a sound/music card).
Please note that there are very few games designed specifically for Gravis Ultrasound but you might be interested in checking out Ultrawolf, it's modified version of Wolfenstein 3D with support for more sound channels, but they added ugly music for unspecified reasons which I wasn't been able to disable. Adlib music remained intact and it can be disabled, the Gravis music not (perhaps it's possible to get rid of .MOD files) http://www.pc-freak.net/blog/enable-gravis-ultrasound-dosbox-enhanced-music-experience
4. Roland Sound Canvas
DOSBox don't have music banks for that, but you may buy Sound Canvas VA for only $125 for native Sound Canvas 55, 88 and 88Pro. https://contentstore.roland.com/software/detail/sound_canvas_va
Alternatively you may check out freely available custom VA sets, I found one which sound like Sound Canvas 55 for free still I'm planning to buy Sound Canvas VA eventually but it's DRM heavy.
But you can't just use it out of the box, you'll need to install additional software and the best would be loopmidi: https://www.tobias-erichsen.de/software/loopmidi.html Please note that this will add additional MIDI device
to your PC so you need to change it before you start playing superb full on General MIDI games, some of old games like Dune 2 doesn't list General MIDI in their setup but recognize Sound Canvas.
Here is a link to our fellow GOGer with instructions on how to install it properly. https://www.youtube.com/watch?v=WumRboSfn90
Also I highly recommend to use Putzlowitschs Vista-MIDIMapper for easy MIDI device change (restart of the MIDI affected program is needed). It works for Vista and Windows 7, and it should work for Windows 10 as well. There are many other free MIDI mappers but this is the most tiny and friendly I've ever tried. https://putzlowitsch.de/wp-content/uploads/2009/01/plw-vista-midi-mapper_0_93.zip
5. Yamaha S-YXG50
It's like Roland Sound Canvas but better, a few games may sound odd in comparison to Sound Canvas.
There are no official VST for this card but you may check what one guy managed to get out of old commercial version: http://veg.by/ru/projects/syxg50
6. Turtle Beach
This I will not comment, it is merely an example of MIDI options.