There's been a few guides out there on how to get the Roland MT-32 midi working with DOS game but this is my favorite and I knew I had to hare it.
https://www.youtube.com/watch?v=qsGQ6AXzen0

I've tested this with both D-fend and GOG DOS games all you have to do is set midiconfig to match the MT-32 sinth and you're good to go.

I like this because you don't need to install any weird drivers or anything and the DOS games seem to recognize the the midi driver. Tested on Windows 7 64Bit.

Thanks for the active GOG member who made the video. :) Albeit I must say, I generally dislike video guides and articles, unless there is a very good reason for it to be in video format (e.g. showing gameplay video or something). I wish there would be a simple text article on some web page explaining the same. Could be in the description section of the video. I think you still need to install the Munt MT-32 drivers, right?

Does it work for all DOSBox games with one change, or do you need to change it one by one for e.g. each of your GOG DOSBox games (with MT-32 support)?

I guess it depends how you install and play your games. I use BASSMIDI (or Coolsoft synth, basically the same thing) anyway because it lets me use superior soundfonts for games with General MIDI music. And at the same time, that same utility also lets me change the sound output to MT-32 (Munt) when needed. So since I have that software installed already (for an improved General MIDI support in DOSBox games), I just use it also for enabling and disabling the MT-32 support, depending which game I am going to play. Works with all games.

Different solutions to the same problem...

I've tried without success to set up Roland MT-32 emulation for several games previously. I think I'll use this to give it another go.

Thanks for posting.

Nope on the drivers :) I wasn't comfortable installing windows drivers for an old piece of hardware I didn't own. Dos now detects munt and all you need to do is tell it to use it via ID number and set the game to use it.

As for your second question you'll need to set each game to use it in both the games native setting and in the dosbox config file. Simply set your game to use MT-32 and place the ID number for the midi into your dosbox config file. You can also try use D-Fend front end which might be a bit easier. It also detects munt.

Well, obviously you still need the Munt MT-32 drivers installed. :)

This is a good option in case you for some reason don't want to install e.g. BASSMIDI (which is not that different from installing Munt), but I want better General MIDI music in my DOS games than what the basic MS wavetable synth can offer, and for that I need BASSMIDI installed anyway.

You should be able to do that without installing any drivers. Munt makes Dobox see the Roland sinth. So after that you should be able to do it by simply making the Roland your default sinth in Dosbox and general midi should sound a bit different. I won't say better because that's a matter of opinion.

I meant that when you install Munt, you are installing drivers. It is the Munt MT-32 drivers that get installed when you install Munt. The drivers provide the MT-32 emulation that you tell DOSBox to use.

Oh okay cool. When I first used munt I had to install windows drivers to use the virtual device which was a pain.