I'm using the MT-32 Emulator from MUNT (latest version 1.0) that installs as a MIDI device in Windows to play music (I also note that the same thing happens when using scummvm's built in MT-32 emulator).
In scummvm, I've configured Scummvm to use the MT-32 Emulator (Windows) MIDI device, and I've also checked the options "Override global MT-32 settings" and "True Roland MT-32" so that scummvm sends MT-32 midi music to the emulator. I watched the introduction of the game and while the music *is* MT32 music, I noticed that in some sections it doesn't sound completely right, as if there are some instruments missing.
To compare, I ran the game in Dosbox and also configured Dosbox to use the same MIDI device (MT-32 Emulator in Windows). In dosbox, the music sounds right throughout the introduction.
So, it appears to me that the engine in scummvm is not sending all MT-32 midi commands to the midi device.
I tested this in ScummVM 1.6.0git2489-g45f95cb (Jan 15 2013 09:44:26) win32
Conquest of Longbow (English version)