I'm not going into the whole history of MIDI on computers and how the MT-32 came to be one of the choices for in game music, nor am I going to elaborate much on the differences between real (or intelligent) mode vs dumb (or UART) mode MPU-401 MIDI interfaces (you can read up about that on Wikipedia).

But, in order for a PC to communicate with an external MIDI device, you require an interface and that interface was originally the Roland MPU-401 MIDI interface. It consisted of an 8-bit card that plugged into an available ISA slot, which then connected to a breakout box - this was later integrated a bit more.

Many games released between 1987 and 1993, that had in game music, supported the Roland MT-32 (and later on its compatible modules, the CM-32L & LAPC-I).

Since the original MPU-401 MIDI interface was a real (or intelligent) mode MIDI interface, some games from that era required such an interface.

Most sound cards released since 1993 had a dumb (or UART) mode MIDI interface (earlier sound cards did have MIDI ports, but not MPU-401 compatible ones), eliminating the need for a separate MIDI interface card, but this was only useful for games that didn't require a real mode MPU-401 MIDI interface.

This is where SoftMPU comes in, since it emulates an intelligent mode MPU-401 MIDI interface via a standard sound card that has a built-in dumb (or UART) mode MIDI interface.

All General MIDI based games (which is when you would use the SC-55) do not require a real mode MPU-401 MIDI interface and therefore also not SoftMPU. The same applies to those MT-32 supported games that also doesn't require a real mode MPU-401 MIDI interface.

Check out the SoftMPU site for a list of supported games and there is also a Wikipedia page of games that supports the MT-32 and which ones require a real mode MIDI interface.

Regardless, in order for any game to communicate with an external MIDI device, you require an MPU-401 MIDI Interface (since that was the standard). Luckily, most sound cards since 1993 had one built in.
You would, in such a case, connect a MIDI cable from the sound card's MIDI/Game port to the external MIDI device and then from the external MIDI device either route the sound output back into the Line In on the sound card, to a mixer or an amplifier.

Take note that some sound cards also came with a "Wave Blaster" header, but that is a different discussion (you can also read up about that on Wikipedia).

As for the sound card, thete are many options. I favour one of the third generation Aztech Sound Galaxy or OEM models with the AZT-2316 chipset. But, you can also look at ESS Audiodrive sound cards.

I see, I think I understand now. So the big MPU-401 unit (the breakout box) is essentially built into the cards themselves then. That was really confusing me.

Thank you very much for taking the time to write this response. I will dig deeper into the things you mentioned and take a look at your recommendations. I appreciate the help very much!

Buy a hardMPU or musicQuest clone from some good people here.

kolderman wrote on 2020-04-26, 05:29:

Buy a hardMPU or musicQuest clone from some good people here.

Not necessary if you have 386DX or faster hardware capable of running SoftMPU.

You can just start with softmpu as stated if your system allows it and then decide if you want to keep using it or go to some hardware solution. That's solid advice IMHO unless your system is a xt/286/slow386 in which case a hardware solution is needed.
But just gauge things and take it from there, there is no "right" answer and it all depends on how you want to build your system , you might want to go hardware all the way or don't mind software drivers - that's all personal preference.
You only need the full intelligent mode for MT-32 games, I do not remember any GM/SC-55 games needing it , they are happy with the simpler UART mode mpu almost every soundcard supports. Ofcourse if you get a dedicated mpu of any kind then you'll just connect both mt-32 and sc-55 to it and call it a day since both mpu modes will be supported.

I have been unable to get Gabriel Knight GM working in DOS without SoftMPU in the past for whatever reason that may have been but instances where a dumb midi interface wont work do exist, possibly platform and/or sound card değendent though..

appiah4 wrote on 2020-04-26, 09:42:

I have been unable to get Gabriel Knight GM working in DOS without SoftMPU in the past for whatever reason that may have been but instances where a dumb midi interface wont work do exist, possibly platform and/or sound card değendent though..

That is interesting - Gabriel Knight ran on Sierra's SCI.2 engine by which time they had already started to develop their games to support General MIDI (with an MT-32 driver provided in some cases for backwards compatibility).
It should therefore be able to run via a standard dumb (UART) mode MPU-401 MIDI interface.

jesolo wrote on 2020-04-26, 09:51:

appiah4 wrote on 2020-04-26, 09:42:

I have been unable to get Gabriel Knight GM working in DOS without SoftMPU in the past for whatever reason that may have been but instances where a dumb midi interface wont work do exist, possibly platform and/or sound card değendent though..

That is interesting - Gabriel Knight ran on Sierra's SCI.2 engine by which time they had already started to develop their games to support General MIDI (with an MT-32 driver provided in some cases for backwards compatibility).
It should therefore be able to run via a standard dumb (UART) mode MPU-401 MIDI interface.

I seem to be misremembering indeed..

Thread here: Gabriel Knight audio problems

I eventually solved this but I don’t remember how..

appiah4 wrote on 2020-04-26, 10:49:

I seem to be misremembering indeed.. […]
Show full quote

jesolo wrote on 2020-04-26, 09:51:

appiah4 wrote on 2020-04-26, 09:42:

I have been unable to get Gabriel Knight GM working in DOS without SoftMPU in the past for whatever reason that may have been but instances where a dumb midi interface wont work do exist, possibly platform and/or sound card değendent though..

That is interesting - Gabriel Knight ran on Sierra's SCI.2 engine by which time they had already started to develop their games to support General MIDI (with an MT-32 driver provided in some cases for backwards compatibility).
It should therefore be able to run via a standard dumb (UART) mode MPU-401 MIDI interface.

I seem to be misremembering indeed..

Thread here: Gabriel Knight audio problems

I eventually solved this but I don’t remember how..

Wasn't there a patch distributed by Sierra themselves to address issues with SCI games running an external synth through an SB16's MPU-401 ?

EDIT: It wasn't a patch it was small executable you ran before starting the game . I cannot remember what it was called . Might not even have been released by Sierra after all .
EDIT2: I was thinking about SBMPU.EXE , so unrelated . Sorry for the noise .