Also, Doom sounds different on my MT-32 than it sounds on some other people MT-32 on youtube

are you sure that all the games you try support MT-32?
Doom and ROTT for example do not support it... even in GM mode the mt-32 will sound weird 🤣 as it's not a GM device or a Sound Canvas one.

MT-32 sounds best in it's own stuff.
GM devices sound best in GM stuff.
Sound Canvas devices sound best in GM and Sound Canvas specific stuff.

The mt-32 can emulate a GM device and a GM/SC device can emulate the mt-32 , results however vary vastly...

Dune 1 sounds *terrible* with the MT-32! The reason being is they did a very poor port of the adlib music; I was horrified when I first heard it.
It was designed for the Adlib Gold (and is one of the few games to support the optional surround sound module). While there are still a handful of Adlib Golds out there, the surround sound modules are few and far between. But yeah that game is meant to be played with OPL3.

When you were using the MT-32 / GM options in DOSBox, I bet it was using the soundcanvas mode instead of MUNT at times; that would explain the 'variances'.

I use a real retro PC in Win98SE's pure DOS mode.

I tried Dune with MT-32 and I got a lot a weird sounds not the music.
I know that it sounds better with OPL3 but I want to understand why it is not working for me.
With King Quest VI it's nearly the same.

Doom, Doom II, Raptor, Comanche, Geoff Grammond Grand Prix sound good; Dune II and Speedball II are very nice...

I use an ISA AWE64 with a real Roland MT-32 Rev0. A220 I5 D1 H5 P330 E620 T6
I have also a Roland M-GS64 (SC-88) working great with Doom and other GM Game.

Also I failed using SoftMPU v1.9

I use SoftMPU v1.9 now and Dune's music is now correct.
Nevertheless I think the FM version is better than MT-32 one.

The audio plugs are in the back 😀

You will need to get the right cables to connect it to whatever speakers you use.

Cannot get enough of your advice pcl! 😁 whatta guy.