You're operating under a few correct conceptions, but also a few misconceptions as well. Let's see if I can clear them up.
Sierra was the champion of the MT-32, yet, something seems off with the MT-32 option in Phantasmagoria.
This is correct - for the late 80s and early 90s. Phantas however was written close to the mid 90s, around 94 (release in 95). In 1992 already, Sierra had switched MIDI devices to the Roland SC-55 (GM). Phantas falls into the category of "later SC-55 (GM) games where Sierra didn't really put any effort into MT-32 conversion/support", as does KQ7, SQ6, LSL6 and a few later games. So it's not surprising that it sounds poor with the MT-32 in large parts.
Mainly, the opening portion of the chants is practically inaudible - if you really crank up the volume, it sounds almost like a whisper or a wind sound. Maybe that's how it's supposed to be? The rest of the composition seems fine, but I am not so sure.
What does "the chants" mean? I assume you don't mean the digital song "Consumite Furore", so I'm guessing some early MIDI piece in the game? I've never played the game, but I do have the master MIDI and digital files for the soundtrack, so I'm fairly familiar with it and the background of its composition.
Please let me know which chapter it falls in, too, so I can try and duplicate it. Save files of course welcome, unless it's right at the start or something.
Additional question - are you playing in DOS or Windows? I'm thinking DOS, since you've mentioned install options. If I'm correct, when you say "MT-32 option", you mean the MT-32 driver in the install program, right?
The weird part comes in if I use Roland Sound Canvas VA (I set it to SC-55 Map). It sounds exactly the same if I use General Midi or MT-32 options. It makes me think that the MT-32 option is broken as these two options don't sound identical on other games in SCVA (nor should I expect it to). Master of Orion is another game where the Roland MT-32 option sounded terrible on MT-32, but sounded correct on Sound Canvas VA.
SCVA is known to have several bugs. GM and MT-32 map sounding the same is a mistake, they have completely different instrument maps, so there's no possibility to "sound the same". For example, Soprano Sax in GM is something like Slap Bass in the MT-32 with regards to program numbers.
They might by accident sync up on a track or two because a few programs have the same instrument number, like pianos and strings, but beyond that..
Additionally, "MT-32 mode" in a SC-55 (or SCVA) is NOT the same as the MT-32 for Phantas, either in terms of instrument maps or waveforms. Not sure if you know, but the MT-32 is a synthesizer, it uses both waveforms and synth parts in tandem with custom/default instruments to create sound. The SC-55 is a ROMpler, it uses predominantly waveforms to create sound.
A SC-55 in MT-32 mode can't play MT-32 custom instruments, only the 128 default. Therefore, it cannot play any MT-32 game which uses anything but the default instruments in the default order (eg Legend of Kyrandia). Phantasmagoria/any Sierra GM game does not use this, it has a custom bank for GM approximation. So no Sierra game is compatible with SC-55's MT-32 emulation, whether MT or GM originally.
This has nothing to do with Sierra's MT-32 driver, of course. Chances are the MIDI files for GM and MT-32 playback are unchanged or close to it (i.e. the GM compositions have minimal changes for the MT-32 "version"), so that's probably a large part of why it sounds sub optimal on the MT-32, it wasn't optimized.
I believe the game itself was composed using an SC-55, so using SCVA is the closest thing I will get to the "as intended" sound without using real hardware; but I find it generally curious that the MT-32 option for this game works perfectly on SCVA as well.
Correct, and correct. Composed with a SC-55. SCVA or the soundfont "Demester SC-55" are probably the two best I've found.
I don't believe that the MT-32 option is working perfectly, it's very probably a mistake. Chances are it isn't using the MT-32 option but has used the GM map instead.
I am not expecting the MT-32 version to be superior to the Sound Canvas version at all, I am just wondering if this game has an error with the MT-32 option out of the box. It wouldn't be the first time a game needed an MT-32 patch to fix utilizing the hardware. 😀
Good. 😉 There's no Sierra GM game that has a superior MT-32 version. Which is logically intuitive, I guess!
I doubt it has an error, although late GM games are basically unplayable on the MT-32, which could be seen as a lack of support or bug. It IS weird that Sierra abandoned the MT-32 pretty much by 1993, when a tiny bit of effort would have made games way more playable with it.