So, I was looking through the internet for the sound tool that was used in a bunch of Game Boy Color games back in 1999/2000/2001,
especially ones by Tiertex, and I actually found the tool.
Conveniently, the composer of various of these games shared the source files of his work online.
I was able to open them in MusyX and "compile" ROMs, but these ROMs are the ones the program call "Slave" ROMs,
they're like how you're able to connect between MIDI inputs and LSDJ, for example, but it sends the MIDI data between MusyX and the
"slave" ROM. However, there is, on a folder, an "example" ROM, which has built-in music... but you're not able to create "example" ROMs,
just "slave" ones... which is unfortunate, since I wanted this to work on an emulator... and the real MusyX Soundtool package might not even exist anymore, so you couldn't do the link between a gameboy and the program... (unless the thing you use to connect MIDI input to LSDJ is the same as MusyX's?)
There's not much information about MusyX Soundtool in the internet, anyways...

(Extra Information: Without MusyX, I was already able to "replace" music in some of these games, by using an hex editor, since the source files also came with a simple compiler that compiled the project into a .pool file, which was exactly the whole hex data for the audio system in the game...)