As someone who likes sound modules and vintage synths quite a bit, I do enjoy having them around for both games and musical tinkering, so I heartily support the endeavour. Before you delve into this, however, I recommend figuring out specific games you wish to savour the MIDI music options on. That way we can better determine ideal options.
For western DOS and GM Win9x games, and assuming sound modules are the preferred form factor, I would recommend a two-module configuration:
(1) Games with MT-32 soundtracks (1987-~1993): Roland MT-32 (Old) and/or CM-32L
(2) Games with GM soundtracks (~1993 and later): Any of the following:
- Roland SC-55 (any model including mkIIs, STs, 50s, etc.)
- Roland SC-88 (or compatible)
- Yamaha MU50
- Yamaha MU80
- Dreamblaster S1 + DreamFace S1 (if using it as an external module)
Ýou will also need a way to connect the system to the device(s). On a vintage system, gameport - MIDI cables (combined with a good sound card) are the typical option, combined with a MPU-401 or SoftMPU as needed. If using DOSBox, by contrast, USB-MIDI adapaters are more popular. Then, the audio is often mixed into the sound card's line-in, or combined with the computer's audio output in either the module's inputs or a separate mixer.
I'll also elaborate on the various standards and modules a little more, in regards to their gaming importance. Apologies if this seems a bit excessive info-wise.
Roland LA: (I recommend referring to Phil's excellent MT-32 guide as well)
MT-32 Old: Recommended for early MT-32 games that expect the 40ms message delay this version requires.
MT-32 New: Headphone jack included. Vibrato is notably faster, so this is not a popular choice.
CM-32L: Recommended for MT-32 games that also employ the added SFX this module has - which are quite a number of them.
CM-32P: PCM only; don't bother.
CM-64: The added PCM channels don't do much in DOS gaming, if anything at all. But for Japanese vintage computer gaming, having one is useful!
CM-500: See CM-64 above, SC-55 below, and also add the fast vibrato of the newer MT-32.
General MIDI Level 1:
There are tons of modules, sound cards, daughterboards, etc. that will support the standard, but the implementation will vary considerably.
A popular, recent, and quite affordable option is the Dreamblaster S1 daughterboard (link to Phil's review); I can't really comment further on it as I don't have one.
SC-55 (non-mkII): Sounds great, and de facto standard. Some versions are not GM-compatible technically.
SC-55mkII: Incremental update to SC-55 with GM support. Going by my 55ST (basically a mkII minus screen and controls, meant for PC games more than music composition), the patches balance and complement each other very well, and given the broad support for the module, this is an excellent choice for GM and GS titles.
SC-88: Considerably more polyphony and a mostly-compatible SC-55 instrument bank option make the 88 a great choice as well. Quite a few MIDI games will like the extra legroom - less note cutoff is usually good.
SC-88Pro and anything higher: I can name Japanese titles that use the 88Pro's capabilities, such as some of the older Windows Touhou games, but I am not aware of any western ones.
I don't recall many games - especially western ones - whose soundtracks are designed with XG support explicitly in mind. That being said, XG-compatible daughterboards and sound modules are also popular, readily available choices.
Many of the Yamaha modules and other devices are able to emulate GS (often called TG-300B mode on the MU line) or have it implemented. It is quite serviceable, but Yamaha uses different samples as well. Whichever sounds better is left to subjective hearing and opinions.
CM-64, FB-01, SC-55ST, SC-8850, SD-20