Not related to the synth.dump error.. But the errors below it seem to indicate that the soundfont your trying to use is missing instruments. It is probably not (fully) General MIDI (GM) compatible. Try using a soundfont such as FluidR3_GM.sf2 it is included in most Linux distributions, and typically...

Just want to point out two SCUMM interpreter versions missing from https://wiki.scummvm.org/index.php/SCUMM/Versions The Secret of Monkey Island version 5.0.18 Available as an updated MONKEY.EXE to solve SoundBlaster playback on 486 or faster systems. https://www.pcgamingwiki.com/wiki/The_Secret_of_...