Thanks!! So these are ready to go, just put them in the game folder?

Yeah, just put them in there and you should be good to go

Thank you :)

Does the game have enhanced soundtrack now (running via SCUMMVM) ?

You still need to go through the process to produce the soundtrack files with proper names from the one included with the GOG release, as described in the messages above.
I followed the solution by Mobeeuz

I also renamed the final folder with the mp3 files from SCUMMVM to "MusicOST" and copied it in the folder where GOG installed the game.

As the final step I set the "extras path" for the game via ScummVM to be this new "MusicOST" folder, and run the game from the "Launch I Have No Mouth, and I Must Scream" shortcut

My "ihnm.ini" file, when opened with a text viewer (eg. Notepad++), under the [ihnm] section looks like this:

[ihnm]
description=I Have No Mouth and I Must Scream (DOS/English)
mute=false
speech_volume=242
voices=true
kbdmouse_speed=3
savepath=d:\games\i have no mouth\saves
extrapath=d:\games\i have no mouth\MusicOST\
path=d:\games\i have no mouth
subtitles=true
gameid=saga
language=en
gui_saveload_last_pos=0
sfx_volume=192
music_volume=192
platform=pc
aspect_ratio=false
guioptions=noAspect lang_English

This was tested on Windows 10 x64 with the scummvm version that the game comes with on GOG (ie. ScummVM 2.0.0) and also a recent development build of ScummVM (2.6.0git).

Unfortunately, I am having issues myself, could you guys perhaps include a Youtube tutorial? Specifically using the ogg files Аллард kindly shared with us, =)