I got it working now, and I even figured out something else; apparently the last music card setting for install.exe makes Ultima 8 use the computer's own MIDI instruments, instead of the synth tones. Thanks for the help!

This is weird. I decided to play Ultima 8, but even though I felt like starting from the beginning, loading the "beginning" save slot just reloaded where I was at the last save, even though I removed everything from the SAVEGAME directory. After that, I just deleted it and re-extracted the archive. …