Never visited the Scummvm forums, but I lurk on this one.
The only thing I know about the Loom CD is that the music (both PC CD and FM-Towns CD), the audio drama (and I also assume all the speech) was done at Earwax studios. Same place that did the FM Towns soundtrack for Zak McKracken as well and I think MI1 CD was done there too (Earwax is credited in the opening credits, but they are for the floppy version too so...).
No idea what kind of hardware they used to create the music.
The CMI MIDIs floating about are probably from the former Mojo hosted site Highland Productions. They were made by listening to the music. CMI used both sampled instruments and live musicians.