2. Open up sshock.exe in HxD and search for "6A 00 6A 00 6A 00 6A 00" The offset for this should be 000019C0
3. Set the last two digits to match the ID of your MIDI device. In my case it was #5 so it now reads "6A 00 6A 00 6A 00 6A 05
Thank you so much for this! This is exactly what I was looking for.
However in my case I could not find the sequence which you indicated at offset 19C0, instead it was at 1591.
I found that changing MIDI device only changes in-game music and in-menu music. All other sounds are generated not via MIDI. So it does make a little difference, but not much.
Apparently the name of sshock.exe file does not matter. So I renamed a copy of the original file sshock-midi00.exe and the modified version sshock-midi01.exe and I can run either file without renaming it back to sshock.exe and the game still works properly. (Of course, the default shortcuts won't work and something else may break down the road, but then I'll just use the default name sshock.exe).